2048 · laguna-s-2.1

Let me analyze this task carefully:

1. I need to build a 2048 game in a single index.html file
2. The game is 4x4 grid
3. Key requirements:
   - Arrow keys slide tiles
   - Adjacent equal tiles merge once per tile per move
   - Score increases by merged value
   - Examples: [2,2,2,2] left → [4,4,0,0]; [2,2,4,0] left → [4,4,0,0]
   - Start with two tiles
   - After a move that changes the board, spawn one tile: 2 (90%) or 4 (10%)
   - Don't spawn after unchanged move
   - Animate movement and merging
   - Prevent input corruption during animation
   - Prevent arrow keys from scrolling
   - Show score
   - Win message on reaching 2048, allow continuing
   - Game Over only when no valid move remains
   - New Game resets board and score

Let me think about the merge logic carefully. The classic 2048 merge:
- For each row (when moving left), process tiles:
  - Slide all non-zero tiles to the left
  - For adjacent equal tiles, merge once (left tile merges with right tile, then skip the next)

Actually, the standard approach is:
- For a row like [2, 2, 4, 0]:
  - First compact: [2, 2, 4, 0] (already compact)
  - Merge: [4, 4, 0, 0]
  - Compact: [4, 4, 0, 0]

Wait, let me reconsider. The standard 2048 algorithm for a row when moving left:

Actually the merge logic: iterate through tiles, when you find two adjacent equal tiles, merge them, and skip the next.

Let me think about [2, 2, 2, 2]:
- Position tiles: [2, 2, 2, 2]
- Merge first pair (positions 0,1): 2+2=4, skip next
- Next pair (positions 2,3): 2+2=4
- Result: [4, 4, 0, 0] ✓

For [2, 2, 4, 0]:
- Merge (positions 0,1): 4
- 4 at position 2 has no equal adjacent
- Result: [4, 4, 0, 0] ✓

So the algorithm per row:
1. Compact (remove zeros, slide to edge)
2. Merge adjacent equals from the edge direction, each tile merges at most once
3. Compact again

Let me write the merge logic properly:

```
function slideRow(row) {
  // compact
  let result = row.filter(v => v !== 0);
  // merge
  for (let i = 0; i < result.length - 1; i++) {
    if (result[i] === result[i+1]) {
      result[i] *= 2;
      result.splice(i+1, 1);
      // don't increment i, so we don't re-merge
    }
  }
  // pad
  while (result.length < row.length) result.push(0);
  return result;
}
```

Let me verify [2,2,2,2]:
- compact: [2,2,2,2]
- i=0: result[0]=2 == result[1]=2 → result[0]=4, splice(1,1): [4,2,2]
- i=0: result[0]=4 != result[1]=2
- i=1: result[1]=2 == result[2]=2 → result[1]=4, splice(2,1): [4,4]
- pad: [4,4,0,0] ✓

[2,2,4,0]:
- compact: [2,2,4]
- i=0: 2==2 → 4, splice: [4,4]
- i=0: 4==4 → 8, splice: [8] ✓

Wait, that gives [8,0,0,0] which is WRONG. The expected is [4,4,0,0].

Hmm, so the issue is that after merging the first pair, the next tile (4) should NOT merge with the merged tile (4).

Wait no. Let me re-examine. [2,2,4,0] moving left:
- The two 2's merge into a 4.
- Then the 4 (original) should merge with the new 4?

But the example says [2,2,4,0] left → [4,4,0,0]. So the 4 and the merged 4 do NOT merge.

This is the classic "merge once per move" rule but applied as "a tile that just merged cannot merge again."

So the rule is: when you merge two tiles, the resulting tile cannot merge again in the same move. But also, in the standard 2048, the cascading behavior is:

Actually, let me think about this differently. The standard 2048 game DOES cascade! [2,2,4] moving left... let me think. In real 2048, [2,2,4] would become [4,4] because 2+2=4, then 4+4=8? No wait.

Hmm, actually in real 2048, [2,2,4] moving left gives [8] because they all cascade? No, that's not right either.

Let me recall the actual 2048 behavior. In 2048, when you move left on a row [2,2,4]:
- The 2's merge into a 4.
- Does the new 4 merge with the existing 4?

Actually in the ORIGINAL 2048, the answer is YES they cascade... no. Let me think very carefully.

The original 2048 algorithm by Gabriele Cirulli processes tiles in a specific way. Let me recall...

The actual 2048 logic: it processes each row, and merges the FIRST pair it can find, then continues. The key is that a merged tile cannot merge again in the same move.

Let me trace the original algorithm for [2,2,4] moving left:
- Compact: [2,2,4,0]
- Now merge from left: i=0, result[0]=2 == result[1]=2, so merge → result[0]=4, result becomes [4,4,0,0]... wait but then do we continue?

In the original algorithm, after a merge, the index advances PAST the merged result. So:

```
compact row
for i in 0..n:
  if i and i-1 are equal and not yet merged:
    merge
```

Actually, the canonical implementation:

```
function move_left(row):
  # slide
  new_row = [x for x in row if x != 0]
  # merge
  for i in range(len(new_row)-1):
    if new_row[i] == new_row[i+1]:
      new_row[i] *= 2
      del new_row[i+1]
      # i is NOT incremented, but because we deleted i+1, next comparison is i vs i+1's new position
  # but wait, this would let the merged tile merge again
```

Hmm, this is the subtle part. Let me think about the actual 2048 behavior with [2,2,2,2]:

Original 2048: [2,2,2,2] left → [4,4,0,0]. Yes this is the example given.

And [2,2,4,0] left → [4,4,0,0]. The example says this.

So in BOTH cases the result is [4,4,0,0]. The difference:
- [2,2,2,2]: first 2+2=4 (merged), then second 2+2=4 (merged). Result: [4,4].
- [2,2,4,0]: 2+2=4 (merged). The 4 doesn't merge with the new 4. Result: [4,4].

So the rule: a merged tile cannot merge again in the same move. This means after merging at position i and i+1 (producing result at i), we skip position i when checking for further merges.

The correct algorithm:
```
compact
i = 0
while i < len-1:
  if row[i] == row[i+1]:
    row[i] *= 2
    remove row[i+1]
    i += 1  # skip the merged tile (move past it)
  else:
    i += 1
```

Let me verify [2,2,2,2]:
- compact: [2,2,2,2]
- i=0: 2==2 → row[0]=4, remove row[1]: [4,2,2], i=1
- i=1: 2==2 → row[1]=4, remove row[2]: [4,4], i=2
- i=2: out of range
- pad: [4,4,0,0] ✓

[2,2,4,0]:
- compact: [2,2,4]
- i=0: 2==2 → row[0]=4, remove row[1]: [4,4], i=1
- i=1: out of range (len=2)
- pad: [4,4,0,0] ✓

The key difference between my first attempt and this: after merging, increment i by 1 (skip past the merged tile) instead of staying at i. Let me re-examine.

In my first version:
```
for i=0 to len-2:
  if result[i]==result[i+1]: merge, splice(i+1)
```
The issue: after splice, length decreases, but i increments. So after merging at i=0 in [2,2,4], we get [4,4], then i=1, and i < len-1 = 1 is false (1 < 1), so loop ends. Result [4,4]. 

Wait, that actually works! Let me re-trace.

[2,2,4,0] → compact to [2,2,4]
- i=0: len-1 = 2, 0 < 2 ✓. result[0]=2 == result[1]=2 → result[0]=4, splice(1,1) → [4,4]
- i=1: 1 < 2 ✓. result[1]=4 == result[2]=4 → result[1]=8, splice → [8]
- pad: [8,0,0,0] ✗

That's the bug. The for loop continues because i increments but the merged tile at position 0 (now 4) could equal position 2 (4).

So the issue is that after a merge, we need to skip the merged tile. The correct approach uses a while loop that skips past merged tiles, OR a for loop that increments i only when no merge, or increments by more after merge.

Let me use this corrected version:
```
function slideRow(row) {
  let result = row.filter(v => v !== 0);
  let scoreDelta = 0;
  let i = 0;
  while (i < result.length - 1) {
    if (result[i] === result[i+1]) {
      result[i] *= 2;
      scoreDelta += result[i];
      result.splice(i+1, 1);
      i++;  // skip the merged tile
    } else {
      i++;
    }
  }
  while (result.length < row.length) result.push(0);
  return {row: result, score: scoreDelta};
}
```

Verify [2,2,2,2]:
- result: [2,2,2,2]
- i=0: 2==2 → result[0]=4, splice(1,1): [4,2,2], i=1
- i=1: 2==2 → result[1]=4, splice(2,1): [4,4], i=2
- i=2: 2 < 3 false → exit
- pad: [4,4,0,0] score=8 ✓ (4+4)

[2,2,4,0]:
- result: [2,2,4]
- i=0: 2==2 → result[0]=4, splice(1,1): [4,4], i=1
- i=1: 1 < 2 (len=2, len-1=1) → 1 < 1 false → exit
- pad: [4,4,0,0] score=4 ✓

So in [2,2,2,2] the score delta is 8 (two merges of 2→4 each worth 4). In [2,2,4,0] the score delta is 4 (one merge of 2→4 worth 4). 

Now to handle all 4 directions, I'll transpose/reverse the board. Moving right = reverse each row, slide left, reverse back. Moving up/down = transpose, slide, transpose back.

Let me structure the code:

```js
const SIZE = 4;

function blankBoard() {
  return Array.from({length: SIZE}, () => Array(SIZE).fill(0));
}

function slideRow(row) {
  // returns {row, score, moved}
  let result = row.filter(v => v !== 0);
  let scoreDelta = 0;
  let i = 0;
  while (i < result.length - 1) {
    if (result[i] === result[i+1]) {
      result[i] *= 2;
      scoreDelta += result[i];
      result.splice(i+1, 1);
      i++;
    } else {
      i++;
    }
  }
  while (result.length < row.length) result.push(0);
  const moved = result.some((v, idx) => v !== row[idx]);
  return {row: result, score: scoreDelta, moved};
}

function reverseRow(row) { return row.slice().reverse(); }
function transpose(board) {
  return board[0].map((_, c) => board.map(r => r[c]));
}

function move(board, dir) {
  // dir: 'left', 'right', 'up', 'down'
  let newBoard = board;
  let totalScore = 0;
  let moved = false;
  if (dir === 'left') {
    for (let r = 0; r < SIZE; r++) {
      const {row, score, moved: m} = slideRow(board[r]);
      newBoard[r] = row;
      totalScore += score;
      moved = moved || m;
    }
  } else if (dir === 'right') {
    newBoard = newBoard.map(reverseRow);
    // ... need to do the slide and reverse back
  }
  ...
}
```

Hmm, this is getting complex. Let me write a cleaner version that handles all directions by rotating.

Actually, let me write a function that moves the board in a direction by transforming, sliding rows left, then un-transforming:

```js
function moveBoard(board, dir) {
  let b = board.map(r => r.slice());
  let totalScore = 0;
  let anyMoved = false;
  
  const ops = {
    left: {reverse: false, transpose: false},
    right: {reverse: true, transpose: false},
    up: {reverse: false, transpose: true},
    down: {reverse: true, transpose: true},
  };
  
  const op = ops[dir];
  if (op.transpose) b = transpose(b);
  if (op.reverse) b = b.map(reverseRow);
  
  for (let r = 0; r < SIZE; r++) {
    const {row, score, moved} = slideRow(b[r]);
    b[r] = row;
    totalScore += score;
    anyMoved = anyMoved || moved;
  }
  
  if (op.reverse) b = b.map(reverseRow);
  if (op.transpose) b = transpose(b);
  
  return {board: b, score: totalScore, moved: anyMoved};
}
```

Wait, I need to be careful about the order of reverse and transpose. For "right": transpose? No. For right, we reverse each row, slide left, then reverse back. So transpose=false, reverse=true. But applying reverse before slide and reverse after slide... let me think.

For right:
- reverse each row
- slide left (compact+merge toward left)
- reverse each row back

So the row that was [a,b,c,d] becomes [d,c,b,a], slide left to compact/merge toward the new left (which is the original right), then reverse back.

Let me restructure. I'll apply transform, slide all rows left in place, then inverse transform.

```js
function moveBoard(board, dir) {
  let b = board.map(r => r.slice());
  let totalScore = 0;
  let anyMoved = false;
  
  // Transform to "slide left" orientation
  if (dir === 'right' || dir === 'down') {
    b = b.map(reverseRow);  // but for down we also transpose
  }
  if (dir === 'down' || dir === 'up') {
    b = transpose(b);
  }
  // Now slide each row left
  ...
}
```

Hmm, the order matters for down. Let me think:
- down: each column slides down. To do this: transpose (so columns become rows), reverse each row (so down becomes left), slide left, reverse back, transpose back.

Actually for "down", transpose then reverse each row, slide left, reverse each row, transpose back.

Let me just carefully define transforms:

- left: slide rows left (no transform)
- right: reverse rows, slide left, reverse rows
- up: transpose, slide left, transpose
- down: transpose, reverse rows, slide left, reverse rows, transpose

Let me verify down with a column [0,2,2,4] (column index):
- This should become [0,0,2,4]? Wait, down means tiles slide to the bottom.

Column [0,2,2,4] sliding down:
- The 2 and 2 merge into 4, the 4 stays. Bottom: [0,0,2,4]? No wait.

[0,2,2,4] moving down: tiles go to bottom. So 2,2,4 go down. The two 2's merge into 4. Result should be [0,0,4,4]? 

Hmm wait. [0,2,2,4] compacted downward: [2,2,4,0] reversed = [0,4,2,2]... let me just use the column.

Actually let me reconsider. Column top-to-bottom: [0,2,2,4]. Moving down means tiles fall to the bottom (index 3). 

Compact toward bottom: [_,2,2,4] → [0,2,2,4] compacted means [0,2,2,4] with zeros at top... 

Let me think of it as reversing the column to make it a row: [4,2,2,0], slide left: 
- compact: [4,2,2]
- merge: i=0: 4≠2; i=1: 2==2 → 4, splice → [4,4], i=2
- result [4,4,0], reverse back to column: [0,4,4,0]... 

Hmm that doesn't match. Let me redo. Column [0,2,2,4], moving down:
- Reverse: [4,2,2,0]
- Compact (remove zeros): [4,2,2]
- Merge left: [4,4] (the two 2's merge into 4, which does NOT merge with the 4 because... wait, after merge i increments past)
  - i=0: 4≠2 → i=1
  - i=1: 2==2 → result[1]=4, splice → [4,4], i=2
  - result [4,4]
- Pad: [4,4,0,0]
- Reverse back: [0,0,4,4]

So column [0,2,2,4] moving down → [0,0,4,4]. That makes sense! The 2's merge into a 4 at the bottom, and the existing 4 shifts down. Wait, but there's a 4 already and a new 4. The existing 4 was at index 3 (bottom), sliding down it stays at bottom. The merged 4 goes to index 2. So [0,0,4,4]. ✓ 

OK so my down algorithm: transpose, reverse rows, slide left, reverse rows, transpose. Let me re-verify it produces [0,0,4,4].

Actually, the cleanest is: transform board so that the target direction becomes "left", slide rows left in the transformed board, then inverse transform.

For down: we want columns to slide down. 
- Transpose: columns become rows. Column [0,2,2,4] (col 0) becomes row [0,2,2,4].
- We want it to slide to the right now (since down in original = right in transposed... no).

Hmm, let me think again. Transpose makes column→row. If tile slides down the column (toward higher index), after transpose that's toward higher index in the row, i.e., right. So for down: transpose, then slide rows right.

Slide right = reverse, slide left, reverse.

So down: transpose, [reverse, slide left, reverse], transpose back. That matches what I said. 

For up: transpose, slide rows left, transpose back. Column [0,2,2,4] → row [0,2,2,4], slide left: 
- compact [2,2,4], merge: i=0:2==2→4 splice→[4,4] i=1, i=1 out of range. result [4,4,0].
Wait, [0,2,2,4] compact left = [2,2,4], merge = [4,4]. pad = [4,4,0,0]. Transpose back: column [4,4,0,0]. 

Column moving up: [0,2,2,4] → tiles slide to top. 2's merge to 4 at top. Result [4,4,0,0]. ✓ 

Now, the score: when 2's merge into 4, score += 4.

Now spawning: after a move that changed the board (moved=true AND/OR merged), spawn one tile. Actually "After a move that changes the board" — so if moved is true (including merges), spawn. If the board is unchanged, don't spawn.

Wait, but moved should be true if tiles moved OR merged. Let me make sure slideRow's `moved` returns true if the row changed (either by movement or merge). My `moved` comparison: `result.some((v, idx) => v !== row[idx])`. This is true if anything changed. Good.

But I need moved to reflect the whole board. In moveBoard, anyMoved = any row's moved. Good.

Spawning: pick a random empty cell, place 2 (90%) or 4 (10%).

Game over: when no valid move remains. A valid move exists if either there's an empty cell, OR two adjacent tiles can merge. So game over when: no empty cells AND no adjacent equals (horizontally or vertically).

Win: when any tile reaches 2048. Show win message but allow continuing.

Let me also handle the "animate movement and merging" without input corruption. I'll use CSS transitions or just animate via a class. Actually, for simplicity and correctness, I could animate using CSS transitions on the tile positions. But that requires knowing target positions.

Given time constraints, I'll do a simpler animation: when a tile moves, I can apply a CSS transition. But implementing smooth sliding with the grid layout... Let me think.

Approach: render each tile as an absolutely-positioned element within a board container. Each cell is at a known (col, row) position. When the board changes, I update each tile's target position and let CSS transition animate it. For merges, I'll handle by keeping the tile element and changing its value with a scale animation, or creating a new tile at the merged position.

This is more complex. Let me consider a simpler but still animated approach.

Actually, given the 15-minute budget and the need for correctness, let me do a reasonable animation:
- Each tile is a div with position absolute, positioned by grid coordinates.
- On move, I compute new positions, set them with CSS transition.
- For merges, the merging tile gets a "merge" animation (pop/scale).

Let me design the rendering:
- Board container (relative positioning), with a visual 4x4 grid background.
- Tile elements: each has data-row, data-col, and we position via transform: translate(col*cellSize, row*cellSize) or left/top.

To animate merges distinctively, I can do: when a tile merges, temporarily show a larger value then shrink. But tracking which tile merged is complex in a grid-render approach.

Alternative simpler animation: I keep tile elements in a 2D structure matching board cells. On each move, for every cell, if there's a tile that needs to move to a new cell, I move the element (CSS transition handles sliding). If two tiles merge into one cell, I show a merge animation (scale up then set value). 

Let me think about how to implement this cleanly with element reuse. This is the tricky part of 2048 animations.

A common approach (like the original 2048): 
- Keep a grid of cells.
- Each tile element knows its current visual position (row, col) and value.
- Also track the previous board to know where tiles came from and which merged.

Actually, let me simplify: I'll animate tile movement by transitioning the tile's position (left/top or transform). For merges, I'll add a temporary merge visual.

Given complexity, let me do this approach:
1. Maintain board state (array of arrays).
2. Render: for each non-empty cell, create/reuse a tile DOM element positioned at that cell.
3. To animate, I track previous positions. When a tile moves from cell A to cell B, I set the element's position to B; CSS transition slides it. 
4. For merges: I can detect a merge if the target cell's new value = sum and source had a tile that moved there merging with another.

Hmm, this detection is fiddly. Let me simplify the merge animation: I'll just use a "merge" class that scales the tile briefly whenever a value appears that was a merge result. Actually I can detect merges by comparing old board and new board:
- For each cell in new board, if value > 0 and it's the result of merging: a merge happened at cell (r,c) if new board[r][c] = 2*v and there were two tiles of value v moving into (r,c).

This requires tracking source tiles, which the move logic can provide if I modify it to track merges.

Let me enhance moveBoard/slideRow to also report which cells had merges. Actually, let me track merge positions.

Let me restructure slideRow to also return merge info... Actually, the merge positions in the board coordinate system. Since I transform the board, the merge positions would be in transformed coordinates. I'd need to map back.

This is getting complicated. Let me take a pragmatic approach for animation:

I'll use CSS transitions on the tile's position. When the board changes:
1. Remove any tiles that no longer exist (or are involved in merges).
2. For tiles that stay/move, update their position with transition.
3. For merges, I'll create the merged tile at the target position with a scale-in animation and a brief highlight.

Let me track tile identities using a unique id per tile object. Board stores tile objects with {value, id}. On move, I reconstruct new tiles: when merging, create a new tile id; when moving, reuse the old tile id. Then by diffing old and new boards by id, I can determine moves (same id, different position) and merges (old id not present in new → it merged into a new tile).

This is a clean approach! Let me do that.

Board state: array of arrays, each cell is null or {value, id}. 

Move logic needs to track merges to create new ids. Let me modify slideRow to work with tile objects and return merges.

Actually, this complicates the core logic. Let me instead keep board as values (numbers), and track ids/positions separately in the renderer by diffing.

Hmm, but to create a NEW id for a merged tile (so the merge animation works), I need to know a merge happened. Let me have slideRow also return the positions (in row-local index) where a merge occurred.

Let me write slideRow to return: {row: values[], score, moved, merges: [indices in result where a merge landed]}

When a merge lands at result index i (the merged tile stays at i after splice), I record i. Then in the transformed board, merge positions are (row i, col mergeIndex). Transform back to original coordinates.

Let me write it:

```js
function slideRow(row) {
  // row: array of numbers
  // compact non-zeros
  const compacted = [];
  const compIndex = []; // original index of each compacted tile
  for (let i = 0; i < row.length; i++) {
    if (row[i] !== 0) { compIndex.push(i); compacted.push(row[i]); }
  }
  const result = compacted.slice();
  const merges = []; // indexes in result where a merge result sits
  let scoreDelta = 0;
  let i = 0;
  while (i < result.length - 1) {
    if (result[i] === result[i+1]) {
      result[i] *= 2;
      scoreDelta += result[i];
      result.splice(i+1, 1);
      merges.push(i);
      i++;
    } else {
      i++;
    }
  }
  while (result.length < row.length) result.push(0);
  const moved = result.some((v, idx) => v !== row[idx]);
  return {row: result, score: scoreDelta, moved, merges};
}
```

Now moveBoard tracks merge positions in original coordinates:

```js
function moveBoard(board, dir) {
  let b = board.map(r => r.slice());
  let totalScore = 0;
  let anyMoved = false;
  const mergeCells = []; // {r, c} in original coords
  
  const reverse = (dir === 'right' || dir === 'down');
  const doTrans = (dir === 'down' || dir === 'up');
  
  // transform
  if (doTrans) b = transpose(b);
  if (reverse) b = b.map(r => r.slice().reverse());
  
  // now slide rows left; merges recorded in transformed coordinates (tc, tr)
  for (let r = 0; r < SIZE; r++) {
    const {row, score, moved, merges} = slideRow(b[r]);
    b[r] = row;
    totalScore += score;
    anyMoved = anyMoved || moved;
    for (const mc of merges) {
      mergeCells.push({tr: r, tc: mc});
    }
  }
  
  // inverse transform
  let mb = b;
  if (reverse) mb = mb.map(r => r.slice().reverse());
  if (doTrans) mb = transpose(mb);
  
  // inverse transform merge cells
  // forward: doTrans then reverse. inverse: reverse back then transpose back
  const unmerge = [];
  for (const {tr, tc} of mergeCells) {
    let c = tc, r = tr;
    if (reverse) c = (SIZE - 1) - c; // because we reversed columns
    if (doTrans) { // transpose: (r,c) <-> (c,r)
      const tmp = r; r = c; c = tmp;
    }
    unmerge.push({r, c});
  }
  
  return {board: mb, score: totalScore, moved: anyMoved, merges: unmerge};
}
```

Hmm wait, I need to be careful with the transpose of merge cells. Let me think.

Forward transform applied to board:
1. If doTrans: B = transpose(B). Now board is transposed.
2. If reverse: B = B.map(reverseRow).

A cell at original (r, c) maps to transformed (tr, tc):
- After transpose (if doTrans): (tr, tc) = (c, r).
- After reverse (if reverse): tc = SIZE-1 - tc.

So forward: if doTrans then (tr,tc)=(c,r); if reverse then tc = SIZE-1-tc.

Inverse (to go from transformed back to original):
- If reverse: tc_orig_in_transverse_frame = SIZE-1 - tc. Let me just invert step by step in reverse order.

Inverse order: first undo reverse, then undo transpose.
- Undo reverse: if reverse was applied, c' = SIZE-1 - tc. (gives the coordinate in the transposed frame)
- Undo transpose: if doTrans, (r_orig, c_orig) = (tc', tr') where (tr', tc') is the position in transposed frame. Wait.

Let me define: position in transformed board is (tr, tc). 
- Undo reverse (reverse was last applied): if reverse, the column in transposed-frame before reverse = SIZE-1 - tc. Call it tc'. Row unchanged tr' = tr. Now position (tr', tc') is in the transposed frame (post-transpose, pre-reverse) if reverse; or directly (tr, tc) if not reverse.

Hmm, I'm conflating. Let me be very careful.

Forward operations in sequence:
- O1: if doTrans, B = transpose(B).
- O2: if reverse, B = B.map(reverseRow).

O1 (transpose): maps point (r,c) in B_pre to (r',c')=(c,r) in B_post. So in the transposed board, what was column c is now row c... the point that ends up at (r',c') in the transposed board came from (c',r'). I.e., B_post[r'][c'] = B_pre[c'][r'].
O2 (reverse rows): maps point (r,c) in B_pre to (r, SIZE-1-c) in B_post. So B_post[r][c] = B_pre[r][SIZE-1-c].

Combined for a point ending at (tr, tc) in the final transformed board:
- After O1 then O2: B_final[tr][tc]. 
  - B_final = O2(O1(B)). 
  - B_final[tr][tc] = O1(B)[tr][SIZE-1-tc] (undo O2) = B[SIZE-1-tc][tr] (undo O1: B_final_afterO1[r][c]=B[c][r], so O1(B)[tr][k]=B[k][tr]).

So the original (r,c) that maps to transformed (tr,tc) is: if doTrans && reverse: r = SIZE-1-tc, c = tr. If doTrans only: r = tc, c = tr. If reverse only: r = tr, c = SIZE-1-tc. If neither: r=tr, c=tc.

Let me just compute the inverse as a function. Let me write inverse transform of a transformed coordinate (tr, tc):

```
let r = tr, c = tc;
// undo reverse (O2 was applied after O1)
if (reverse) c = (SIZE - 1) - c;  // now in O1's output frame
// undo transpose (O1)
if (doTrans) { const tmp = r; r = c; c = tmp; }
// now (r, c) is original coordinate
```

Wait, I need to apply inverses in reverse order: undo O2 first, then O1. 
- Undo O2 (reverse rows): if reverse, the point (tr, tc) in B_final came from (tr, SIZE-1-tc) in O1's output. So now point is (tr, SIZE-1-tc) in the post-O1 board.
- Undo O1 (transpose): if doTrans, post-O1 board[r'][c'] = pre-O1 board[c'][r']. So a point (tr, tc) in post-O1 came from (tc, tr) in pre-O1. 

So: 
```
let r = tr, c = tc;
if (reverse) c = (SIZE-1) - c;
if (doTrans) { const tmp = r; r = c; c = tmp; }
```

Let me verify with down direction on column. For down: doTrans=true, reverse=true. Merge cell recorded at transformed (tr, tc). 

Example: column [0,2,2,4] at original column c=0, rows 0-3: values 0,2,2,4 at (0,0),(1,0),(2,0),(3,0). Merge happens between the two 2's. After transpose, col 0 becomes row 0: [0,2,2,4]. After reverse rows: [4,2,2,0]. Slide left: compact [4,2,2], merge → [4,4] at indices... merges push i=1 (the second position, where 2's merged). result after pad [4,4,0,0]. mergeCells transformed (tr=0, tc=1).

Now inverse: reverse → c = 3-1 = 2. doTrans → swap: r=2, c=0. So original (r=2, c=0). 

But the merged value 4 is at original column 0, and after sliding down, the two 2's (at rows 1,2) merge into 4 at row 2. So original (r=2,c=0) should hold the merged 4. ✓ The column [0,2,2,4]→ down → [0,0,4,4]: row 2, col 0 = 4. ✓ 

So the inverse transform is correct.

Let me double check the reverse-only case (right direction). doTrans=false, reverse=true. Merge at transformed (tr, tc). Inverse: reverse → c = 3-tc. doTrans false. Original (r=tr, c=3-tc). 

For right on row [2,2,0,0] (already at row r=0): reverse → [0,0,2,2], slide left: compact [2,2], merge → [4], pad [4,0,0,0], merge at tc=0. Inverse: c=3-0=3. Original (0,3). The merged 4 should be at right edge (col 3). ✓ Row [2,2,0,0] → right → [0,0,0,4]. Merged tile at col 3. ✓

Now for the animation, I'll track tile ids. Let me reconsider whether the merge animation complexity is worth it. The task says "Animate movement and merging." So I need at least movement animation, and merging should be animated too.

Let me go with the tile-id diffing approach for animation:

Board state: I'll store board as values (numbers) for the logic, but for rendering I'll maintain a separate structure of tile objects with positions and ids. Actually, let me store the board with tile objects {value, id}.

Hmm, but the move logic operates on values. Let me keep board as values for logic, and maintain a parallel "tile registry" for rendering.

Approach:
- `board`: SIZE x SIZE of numbers (0 = empty).
- `tiles`: a map from tileId → {value, r, c}. Plus a 2D reference grid `grid[r][c]` = tileId or null.

Actually, simpler: I'll keep the board as values for logic and win/lose detection, and a separate grid of tile objects for animation. But that duplicates state and is error-prone.

Let me reconsider. The cleanest: board stores {value, id} or null. The move logic needs to merge (create new id) and slide. Let me write the move logic directly on tile objects.

Actually, the merge-detection via slideRow returning merge positions in transformed coords, then inverse-transforming to original coords, gives me merge cells in (r,c) of original board. Combined with the new board values, I know exactly which cells have merged tiles. 

For animation, here's my plan:
- Each cell may have a tile element. I'll track tile elements by id. 
- When board changes: 
  - For cells that have a tile in new board but the id matches an old tile → it's a move (or no movement); reposition with transition.
  - For cells that are merge cells → animate a merge (the merging tiles fly together / the target grows).
  - Remove tiles whose id is not in the new board.

This requires assigning ids. Let me maintain a grid `tileGrid[r][c]` = tileId (or 0), and a `tiles` object: id → {value, el, r, c}.

On move:
1. Compute new board values + merge cells using moveBoard logic (value-based).
2. Assign new ids:
   - For each cell in new board with value v:
     - If it's a merge cell: create a new tile id, value = v (the merged value). The old tiles (the two sources) disappear.
     - Else: this tile came from some old position. Find an old tile with matching value that's moving here. Reuse its id.
   
The tricky part: matching old tiles to new positions to reuse ids (so they slide rather than pop). 

This is essentially the "move old tile to new cell" problem. A clean way: For non-merge cells, find the old tile (by id) that should move to this new cell. But how to pair them?

Standard 2048 approach: I'll pair by tracking, for each move, the source positions. But moveBoard (value-based) doesn't track sources.

Alternative simpler animation that still looks acceptable: 
- On each move, remove all tile elements and re-create them at their new positions with a fade-in? No, that loses the sliding look.

Let me think of a simpler but effective animation. Maybe I'm overcomplicating. 

Option: Animate by repositioning tile elements to new cells. For each new cell:
- If there's an existing tile element at the OLD source cell with the same value and it's not a merge → move that element to the new cell (CSS transition slides it).
- If it's a merge cell → the two source elements "fly" toward the merge cell and we spawn a new (larger) element; old elements removed.

To know sources, let me have the move logic return, per new cell, whether it's a merge and the source cells. That's more info. 

Hmm, given time constraints, let me take a pragmatic middle ground that gives a good-looking animation:

I'll do the id-diffing with a simple heuristic:
- Maintain tileGrid (ids) and tiles (id → element).
- After computing new board (values) and merge cells:
  - First, identify which new cells are merges.
  - For each new tile cell (r,c) with value v:
    - If merge cell: this is a brand new merged tile. Create new element at... the merge point. To animate the merge nicely, I'd want both sources to converge. But I don't have sources easily. 

OK let me just track sources in the move logic. Let me augment slideRow to track source positions and merge sources.

Actually, let me step back and reconsider the scope. The verification steps are:
1. Play a short sequence of arrow-key moves; check tiles slide, a valid move adds a tile, merges update score by merged value.
2. New Game resets to two tiles with reset score.

The animation requirement: "Animate movement and merging without allowing rapid input to corrupt board state." 

I think a reasonable interpretation: tiles should visually move (slide) to their new positions with a transition, merges should show some visual change, and during animation rapid input is disabled. I don't need pixel-perfect 2048-style fly-to-center merge animation. 

Let me do this:
- Tiles are absolutely positioned by cell. 
- On move, update each tile element's position (left/top) to the new cell. CSS transition (e.g., 0.15s ease) makes them slide.
- For merges: the merged tile's value updates to the new (doubled) value, and I add a CSS animation (scale pop). For the two source tiles that merge, they both move to the target cell and one "wins" — but if both move to same cell, that's visually weird (two elements overlap). 

Hmm. Let me handle merges specially: when a merge occurs at cell (r,c) from sources, I'll take ONE source tile element, move it to (r,c), change its value, and add merge animation; remove the OTHER source element. Both sources visually move to (r,c) but I only keep one... they'd both transition to (r,c) then one disappears. That's acceptable-ish.

Actually, the cleanest visual: both source tiles slide to the merge cell, and as they overlap, the value pops to the merged value, then one element is removed. To do this I need to know the two source positions.

OK you know what, let me just track sources. I'll write the move logic to return, for each resulting tile, its source positions (1 or 2 sources). Let me modify slideRow to track this.

Let me redefine slideRow to operate and track sources per result position:

```js
function slideRow(row) {
  // row: array of numbers
  // returns: { row: values, score, moved, merges: [{pos, sources: [origIdx,...]}] }
  
  // collect non-zero with original indices
  const cells = []; // {val, orig}
  for (let i = 0; i < row.length; i++) {
    if (row[i] !== 0) cells.push({val: row[i], orig: i});
  }
  
  const out = []; // result cells: {val, sources: [orig indices]}
  let scoreDelta = 0;
  let i = 0;
  while (i < cells.length) {
    if (i < cells.length - 1 && cells[i].val === cells[i+1].val) {
      const val = cells[i].val * 2;
      scoreDelta += val;
      out.push({val, sources: [cells[i].orig, cells[i+1].orig]});
      i += 2;
    } else {
      out.push({val: cells[i].val, sources: [cells[i].orig]});
      i += 1;
    }
  }
  
  // build result and merges
  const result = [];
  const merges = [];
  for (let k = 0; k < row.length; k++) {
    if (k < out.length) {
      result.push(out[k].val);
      if (out[k].sources.length === 2) merges.push({pos: k, sources: out[k].sources});
    } else {
      result.push(0);
    }
  }
  
  const moved = result.some((v, idx) => v !== row[idx]);
  return {row: result, score: scoreDelta, moved, merges};
}
```

Wait, but here `orig` is the index in the row after compaction, not the original board index. Hmm. Because cells have orig = original index in row. But after transpose/reverse transforms, the "row index" and the index within the transformed row don't directly map back to original board (r,c) easily for sources.

This is getting really involved. Let me reconsider.

Given the time budget (15 min) and that I also need to run/verify in browser, let me simplify the animation to something clean and correct:

**Simplified animation approach:**
- Tiles slide via CSS transition on `transform` (translate) based on cell coordinates.
- For each move: I compute the new board. I maintain tile elements keyed by a generated id per cell position is NOT tracked; instead I maintain a grid of tile objects with ids.
- When board changes, I update ids: 
  - New merged tiles get fresh ids (these are "new" elements that fade/scale in at the merge position).
  - Non-merged tiles keep their existing id from their source (I need to know the source).

For non-merged tiles, the source is the tile that slid into that cell. Since no merge, exactly one source tile maps to this cell. 

I think tracking sources properly is the robust way. Let me just do it correctly with a value+id based board.

**Final decision: Board stores tile objects {value, id, merge boolean} with null for empty. Move logic produces new board with new ids for merged tiles and reuses ids for slid tiles.**

Let me write move logic on tile objects:

```js
function slideRowObjs(row) {
  // row: array of {value,id} or null
  const cells = row.filter(t => t !== null);
  const out = [];
  const merges = [];
  let score = 0;
  let i = 0;
  while (i < cells.length) {
    if (i < cells.length - 1 && cells[i].value === cells[i+1].value) {
      // merge: create new tile object
      const merged = {value: cells[i].value * 2, id: nextId(), merged: true};
      score += merged.value;
      out.push(merged);
      merges.push({obj: merged, sources: [cells[i], cells[i+1]]});
      i += 2;
    } else {
      // keep the tile object as-is (reuse id) but mark not merged
      out.push({...cells[i], merged: false});
      i += 1;
    }
  }
  while (out.length < row.length) out.push(null);
  const moved = out.some((t, idx) => {
    const a = t, b = row[idx];
    if (!a || !b) return a !== b; // one is null and other isn't
    return a.id !== b.id || a.value !== b.value;
  });
  return {row: out, score, moved, merges};
}
```

Hmm, `moved` detection with ids: a tile moved if its id ended up at a different cell index. Let me track positions. Actually, let me compute moved by comparing the id-at-each-cell before and after:

If for any cell idx, the tile id differs from before (treating null as no id), then something moved or merged. Actually that's not quite right either because a tile could stay in place (id same, position same) → not moved.

Let me define moved = the board visually changed, i.e., there's a tile at a position that differs. Simplest: moved if any cell's tile id differs from before. But a merged tile has a NEW id, so its cell id differs → counts as moved (correct, since merging changed the board). And a tile that slid to a new cell has different id at this cell → moved. And a tile staying put has same id → no change at that cell. 

But wait: if tile A was at cell 0 and moves to cell 2, then cell 0's id changes (from A to something else/nothing) and cell 2's id changes (to A). So moved=true. Good. If nothing moves, every cell's id is identical → moved=false. 

But there's subtlety: a tile staying in place has the same id, good. However, if a tile "slides" but lands in the same cell (e.g., [2,2,0] → no, 2's merge). Let me check [2,0,2] left: compact [2,2], merge → [4] new id at cell 0. Cell 0 before id1(2), after id_new(4) → changed → moved. Good.

Let me just compute moved by id comparison:

```js
const moved = out.some((t, idx) => (t ? t.id : 0) !== (row[idx] ? row[idx].id : 0));
```

Wait, but this fails: consider [2,0] (id at cell0 = A) vs after a move where A stays at cell 0 (id A) and cell1 null → no change → moved false. Correct.

Consider a tile that moves from cell0 to cell1 but no merge, no other tiles: row [A, null] → out [null, {id: A...}]. Cell0 id before A, after 0 → changed → moved. Cell1 before 0, after A → changed → moved. So moved=true. Correct (it moved).

Good, id comparison works for moved detection (as long as I always reuse the same id object for non-merged tiles). But I'm creating new objects (spread) which changes reference but I copy id. I compare by id not reference, so fine.

Actually, for non-merged tiles I do `{...cells[i], merged:false}` which creates a new object but same id. The moved check uses id, so a tile staying in place: out[idx].id === row[idx].id → not moved at that cell. 

But here's a problem: for the move to be considered "no change", ALL cells must have matching ids. If a tile slides but I reuse its id at a new cell, the old cell's id becomes 0 and new cell's id becomes that id — both differ from before → moved. Good.

OK this works. Now transform for directions with tile objects: transpose and reverse work on arrays of objects fine (they're just arrays). The `merges` info: each merge has a `merged` obj (in the row's local result) and sources. For animation I'll use merges to know which tiles merged and where.

For direction transforms, after transpose/reverse, the merge positions are in transformed coords; I'll inverse-transform them to original coords, OR I can handle merges purely in the rendering by diffing old vs new board ids. 

Actually, since I track ids, I can detect merges in rendering by diffing: a merge cell in the new board has a tile whose id did NOT exist in the old board (it's a freshly created merged tile). And the source tiles (old ids) that are gone are the ones that merged. But I can't easily pair them. However, for the merge animation, I mainly need to:
- Animate the source tiles moving to the merge cell.
- Then show the merged value growing / a pop.

If I know the merge cell position and the two source positions, I can move both source elements to the merge cell and then reveal the merged value.

The `merges` array from the move logic gives me exactly the source tiles and the merged tile position — but in transformed coordinates. Let me inverse-transform them.

This is getting long. Let me just commit to writing it and tracking merges through transforms with inverse transforms in original coordinates. I'll write helper to transform coords.

Actually, simpler: Let me NOT use transpose/reverse on the tile-object board. Instead, I'll write a generic move that, given a direction, computes the result by processing appropriate lines (rows or columns) in the appropriate order, and records merge source positions in original coordinates directly.

Let me write move logic that processes "lines". A line for left/right is a row; for up/down is a column. For right/down, I process the line reversed and then reverse the result back, tracking original positions.

Hmm, this is also fiddly but more localized. Let me define:

For direction `left`: process each row, line = row, index within line = col index. Merge sources and result positions map directly to (r, c).
For `right`: process each row, but line = row reversed; result reversed back. The position mapping: result position p in the right-slide corresponds to original col (SIZE-1-p)... 

Let me just handle each direction by mapping to a line and an index transform.

I'll define, for each direction, a function that extracts line elements with their (r,c), runs a generic "slide line" that returns new line + merge info with original (r,c), then writes back.

```js
function slideLine(line) {
  // line: array of {val, pos}  where pos is the original (r,c) OR just track index
  ...
}
```

Ugh, let me just write per-direction using a unified slideLine that works on values + tracks result positions and merge source positions, then map positions per direction.

Let me define slideLine to take an array of {value, r, c} (the line cells in the order to be compacted toward the front) and return:
- newLine: array of {value, r, c} aligned to front, with merged tiles as new entries.
- score, moved.
- mergeCells: [{r, c, sources: [[r,c],[r,c]]}] for positions where a merge landed.

Then for each direction, I build lines:

```js
function getLines(dir, board) {
  // returns array of {cells: [{value,r,c}], set: function(newValues) }
  const lines = [];
  for (let i = 0; i < SIZE; i++) {
    const cells = [];
    if (dir === 'left') {
      for (let c = 0; c < SIZE; c++) cells.push({value: board[i][c], r: i, c});
    } else if (dir === 'right') {
      for (let c = SIZE-1; c >= 0; c--) cells.push({value: board[i][c], r: i, c});
    } else if (dir === 'up') {
      for (let r = 0; r < SIZE; r++) cells.push({value: board[r][i], r, c: i});
    } else if (dir === 'down') {
      for (let r = SIZE-1; r >= 0; r++) cells.push({value: board[r][i], r, c: i});
    }
    lines.push(cells);
  }
  return lines;
}
```

Then slideLine compacts/merges the cells array (in front-to-back order for the slide direction), producing new cells aligned to front. Then I write them back to board positions. But writing back needs to map the result cells to actual (r,c). Since each result cell carries its own {r,c}, and the front-aligned result means result[0] goes to the "front" of the line which corresponds to the first cell in `cells` (which has the correct original r,c for the front).

Wait, but after merging, the result cells need new positions: the front of the line. The first cell in `cells` (cells[0]) is the front (e.g., for left, col 0; for right, col SIZE-1). After compaction, result[0] should occupy cells[0].r,cells[0].c. result[1] → cells[1].r,cells[1].c. etc. And emptied positions (after result) get 0.

So writing back: for k in 0..SIZE-1: if k < result.length, board[result[k].r][result[k].c] = result[k].value; else board[cells[k].r][cells[k].c] = 0... 

Hmm, but result cells carry the source {r,c} which after compaction should be reassigned to front positions. Let me have slideLine reassign positions: result[k].r/.c = cells[k].r, cells[k].c (the k-th front position). For merge, the merged result cell takes cells[k] position.

Let me write slideLine:

```js
function slideLine(cells) {
  // cells: [{value, r, c}] in front-to-back order for the slide direction
  // compact non-zero to front
  const compacted = cells.filter(x => x.value !== 0);
  const out = [];
  let score = 0;
  const mergePositions = []; // {r, c, sources: [[r,c],[r,c]]}
  let i = 0;
  while (i < compacted.length) {
    if (i < compacted.length - 1 && compacted[i].value === compacted[i+1].value) {
      const v = compacted[i].value * 2;
      score += v;
      out.push({value: v, r: compacted[i].r, c: compacted[i].c, merged: true,
                sources: [[compacted[i].r, compacted[i].c], [compacted[i+1].r, compacted[i+1].c]]});
      i += 2;
    } else {
      out.push({...compacted[i], merged: false, sources: [[compacted[i].r, compacted[i].c]]});
      i += 1;
    }
  }
  // pad
  while (out.length < cells.length) out.push({value: 0, r: cells[out.length].r, c: cells[out.length].c});
  const moved = out.some((t, idx) => t.value !== cells[idx].value || (t.merged));
  // Actually moved: true if any value changed OR positions changed. Let me compute properly.
  return {out, score, mergePositions: out.filter(t => t.merged).map(t => ({r: t.r, c: t.c, sources: t.sources}))};
}
```

Hmm the `moved` calc. Let me think: moved should be true if the board changes. Given front-aligned compaction, if compacted length == cells.length AND no merges happened AND values identical in order → not moved. Let me compute moved as: (compacted.length !== cells.length) OR any merge OR any value differs. Simplest robust: compare the resulting board line values vs original line values:

```js
const moved = out.some((t, idx) => t.value !== cells[idx].value) || out.some(t => t.merged);
```

Wait, if compacted has same length, no merge, and values identical → out[i].value === cells[i].value for all i, and no merged → moved false. But consider [2,2,4,0]: compacted [2,2,4], merge → out [4,4,0,0]. cells values [2,2,4,0]. out[0]=4≠2 → moved true. Good.

Consider [4,4,0,0] (a stable row): compacted [4,4], merge → out [8]?? No wait, 4==4 merges to 8. That would say moved. But [4,4] SHOULD merge when sliding left. So moved=true is correct (it does change). 

Consider [4,0,0,0] (stable): compacted [4], no merge, out [4,0,0,0]. cells [4,0,0,0]. values identical, no merge → moved false. 

Consider [0,4,0,0] sliding left: compacted [4], no merge, out [4,0,0,0]. cells [0,4,0,0]. out[0].value=4≠0 → moved true. 

So moved detection works.

Now writing back to board: For each line, I have `out` array aligned to front positions = cells[k] positions (out[k] takes cells[k].r,c). And board updated.

Actually, out[k] already has r,c set to the front position (cells[k].r,c for the compacted ones, and padded with cells[out.length].r,c etc.). Let me make sure out's r,c are the FRONT positions, not source positions. In my slideLine, for non-merged out entries I set r,c = compacted[i].r,c (source position) — that's WRONG; it should be the front position cells[k].r,c.

Let me fix: assign front positions. After building out values, set positions:

```js
for (let k = 0; k < out.length; k++) {
  out[k].r = cells[k].r;
  out[k].c = cells[k].c;
}
```

But merges need sources in original coords (which I captured from compacted[i] before repositioning). Let me capture sources before reassigning.

Let me rewrite cleanly:

```js
function slideLine(cells) {
  const compacted = cells.filter(x => x.value !== 0);
  const outVals = [];
  const sourceMap = []; // for each output, the source [r,c] cells
  let score = 0;
  let i = 0;
  while (i < compacted.length) {
    if (i < compacted.length - 1 && compacted[i].value === compacted[i+1].value) {
      const v = compacted[i].value * 2;
      score += v;
      outVals.push(v);
      sourceMap.push([[compacted[i].r, compacted[i].c], [compacted[i+1].r, compacted[i+1].c]]);
      i += 2;
    } else {
      outVals.push(compacted[i].value);
      sourceMap.push([[compacted[i].r, compacted[i].c]]);
      i += 1;
    }
  }
  // build output tiles with front positions
  const out = [];
  for (let k = 0; k < cells.length; k++) {
    if (k < outVals.length) {
      out.push({value: outVals[k], r: cells[k].r, c: cells[k].c,
                sources: sourceMap[k], merged: sourceMap[k].length === 2});
    } else {
      out.push({value: 0});
    }
  }
  const moved = out.some((t, idx) => t.value !== cells[idx].value) || out.some(t => t.merged);
  const merges = out.filter(t => t.merged).map(t => ({r: t.r, c: t.c, sources: t.sources}));
  return {out, score, moved, merges};
}
```

Then the full move:

```js
function moveBoard(board, dir) {
  // board: array of arrays of numbers (0 empty)
  let totalScore = 0;
  let anyMoved = false;
  const allMerges = []; // {r,c, sources:[[r,c],[r,c]]}
  const newBoard = board.map(r => r.slice());
  
  const lines = [];
  for (let i = 0; i < SIZE; i++) {
    const cells = [];
    if (dir === 'left') for (let c=0;c<SIZE;c++) cells.push({value: board[i][c], r:i, c});
    else if (dir==='right') for (let c=SIZE-1;c>=0;c--) cells.push({value: board[i][c], r:i, c});
    else if (dir==='up') for (let r=0;r<SIZE;r++) cells.push({value: board[r][i], r, c:i});
    else if (dir==='down') for (let r=SIZE-1;r>=0;r--) cells.push({value: board[r][i], r, c:i});
    lines.push({i, cells});
  }
  
  for (const {cells} of lines) {
    const {out, score, moved, merges} = slideLine(cells);
    totalScore += score;
    anyMoved = anyMoved || moved;
    allMerges.push(...merges);
    // write back
    for (let k = 0; k < out.length; k++) {
      newBoard[out[k].r][out[k].c] = out[k].value;
    }
  }
  
  return {board: newBoard, score: totalScore, moved: anyMoved, merges: allMerges};
}
```

This tracks merges in original (r,c) coords with both source positions. 

Now `merges` gives me, for each merged tile, the target cell (r,c) and the two source cells. For animation:
- The two source tiles (at source cells) slide to the target cell (r,c). 
- The merged tile appears with the doubled value.

For id tracking / element reuse: I'll maintain a tile element per id. Let me maintain:
- `tileGrid[r][c]` = tileId (or null).
- `tiles` = Map id → {value, el}.

Animation algorithm on move result {board, merges, ...}:
1. Determine new tileGrid from new board (assign ids):
   - For merge cells: create a NEW id for the merged tile.
   - For non-merge cells that have a tile: reuse the id that should be there. But which id? The source id. For a sliding (non-merge) tile, it comes from one source cell. I can reuse that source's id.
2. Build new tileGrid:
   - For each cell (r,c) with value v in new board:
     - If (r,c) is a merge target: newTileGrid[r][c] = newId; this is a merged tile.
     - Else: find the source. The source is the tile that slid here. I can determine source by... hmm. 

Matching sources to non-merge tiles: A non-merge tile at new cell (r,c) came from the nearest source cell in the slide direction. Without explicit tracking, I'll match by id reuse: I'll carry over ids. 

Actually, let me track source→dest in slideLine. Let me also output, for each output tile, its source id (for non-merged) so I can reuse. But ids are in the renderer, not the value-based board. 

Let me restructure: instead of value-based board, use a board of tile objects {value, id} (null for empty). Then slideLine naturally preserves ids for non-merged and creates new ids for merged. This is cleaner!

Let me redo with tile-object board:

```js
function slideLineObjs(cells) {
  // cells: [{tile: {value,id} | null, r, c}]  front-to-back
  const compacted = cells.filter(x => x.tile !== null);
  const out = []; // {tile, r, c, merged, sources:[[r,c]]}
  let score = 0;
  let i = 0;
  while (i < compacted.length) {
    if (i < compacted.length - 1 && compacted[i].tile.value === compacted[i+1].tile.value) {
      // merge
      const mergedTile = {value: compacted[i].tile.value * 2, id: nextId()};
      score += mergedTile.value;
      out.push({tile: mergedTile, r: cells[i].r, c: cells[i].c, merged: true,
        sources: [[compacted[i].r, compacted[i].c], [compacted[i+1].r, compacted[i+1].c]]});
      i += 2;
    } else {
      out.push({tile: compacted[i].tile, r: cells[i].r, c: cells[i].c, merged: false,
        sources: [[compacted[i].r, compacted[i].c]]});
      i += 1;
    }
  }
  // pad
  for (let k = out.length; k < cells.length; k++) out.push({tile: null});
  // assign front positions (out[k] → cells[k] position)
  for (let k = 0; k < out.length; k++) {
    if (out[k].tile) { out[k].r = cells[k].r; out[k].c = cells[k].c; }
  }
  const resultBoard = out; // each has tile, r, c
  return {out, score, ...};
}
```

Then new board tile-objects: for k, newBoard[out[k].r][out[k].c] = out[k].tile (or null).

And merges: for tiles with merged:true, target = (out[k].r, out[k].c), sources = the two source cells.

For the renderer, to animate:
- Old tileGrid (id at each cell) vs new tileGrid.
- For each new cell (r,c) with tile T (id):
  - If T's id existed in old grid at a source cell: it's a move (reuse element), slide from source to (r,c).
  - If T is a merged tile (new id): it's a merge. Animate the two source elements moving to (r,c), then reveal merged value (pop).
- Remove elements for old tiles whose ids are no longer in the grid.

For merges, the sources are old tiles that will be removed. I'll move those source elements to (r,c) and then, after they converge, grow/reveal. 

Let me simplify the merge visual: 
- Move both source elements to target (r,c) with transition.
- When they arrive, kill one element, set the survivor's value to merged value + add "pop" animation (scale up quickly).
- Actually with two elements both at (r,c) overlapping, then one disappears and value shows doubled. 

Given complexity and time, let me do a cleaner merge animation:
- On merge at (r,c) with sources S1, S2: 
  - Take the source element at S1 (reuse its id? no, merged tile has new id). 
  - Actually, let me: remove the S2 element (or both), and take the S1 element, move it to (r,c), change its value to merged value, add a "merge-pop" CSS animation (scale + color flash). The S1 element "was" the tile that won the merge. S2 element flies in and disappears.

Hmm. Honestly, a simpler visually-acceptable merge: 
- Both source elements translate to (r,c).
- Then immediately (or after a tick) one is removed and the tile's value updates to merged, with a scale pop.

I worry about edge cases. Let me allocate time budget: write the code, run it, verify core mechanics (slide, spawn, merge, score, win, game over, new game), and do a decent-but-simple animation.

Let me simplify the animation to: **tile position transitions + merge pop, using id diffing without source tracking for non-merges** — actually I do have source tracking via the tile-object board. Let me just implement the move with tile objects and animate by repositioning existing elements.

Let me reduce risk: I'll implement animation at the position level using element positions and a map. Here's a clean rendering approach:

Render model:
- `tiles` = Map<tileId, {value, r, c, el}>. (el is the DOM element)
- Render: for each tileId, position el at (r,c) via CSS transform translate.
- `placeTile(r,c,value)`: create element, assign id, add to map, append to board, position it.
- On move result:
  - newTiles = Map built from new board.
  - For merges: for each merge {target(r,c), sources}, do merge animation.
  - For moved non-merge tiles: reposition elements.
  - Remove tiles not in new board.

Let me handle it in this order per move result:
1. Compute new board (tile objects) + merges + moved + score.
2. Build a set of new tile ids.
3. For merges: 
   - For each merge m at target cell (r,c) with sources [s1, s2]:
     - Get elements for ids at s1 and s2 (old tiles).
     - Remove those ids from `tiles` map (they're being consumed).
     - Take s1's element, move it to (r,c), set its value to merged value, add id = merged id, add "merge-pop" class. Update map.
     - Remove s2's element from DOM (or animate it flying to (r,c) then remove). 
   - To keep it simple: just remove s2 element, and animate s1 element moving to target with a pop and value change. Actually the merged tile's value doubles; showing the pop communicates the merge.
4. For non-merge tiles that moved: reposition their elements to new (r,c) with transition.
5. For the newly spawned tile: place it (fade in).
6. Remove leftover elements.

For step 3, taking s1's element and changing value + moving + pop is a reasonable merge animation. s2 element disappears. But visually, s1 moving from s1's cell to target while s2 just vanishes is odd if s1 didn't move much. 

Better merge animation without perfect source convergence: 
- Create the merged tile element at the target cell, scale it from 0 (pop-in), set value. 
- Remove both source elements with a fade-out.
- Score label pops.

This is clean: merged tile pops in at target, sources fade out. It's a common 2048 style. Let me do that.

So:
- Merge at (r,c): create new element at (r,c) with value (scale-in animation). Fade out & remove source elements.
- Move (non-merge, changed cell): reposition element to new (r,c) with slide transition.
- Spawn: new element fade/scale in.

For id reuse on moves: a non-merged tile keeps its id and its element; just reposition. Since tile-object board reuses the same id object (I pass the same tile object), I can match by id. 

Let me now also handle: when an element moves because its cell changed, I set transform to new position; CSS transition slides it. When a tile is created (merge result, spawn), I insert it already at final position and trigger appear animation.

Let me now write the move logic producing new board of tile objects + merges + moved, then render.

Let me also handle the "moved" for spawning: only spawn if moved (board changed). Spawn after the move animation completes? The task says "After a move that changes the board, spawn exactly one tile." I'll spawn after animation so it doesn't visually conflict. I'll set a flag and spawn after transition.

Let me also prevent rapid input during animation: while animating, ignore key presses. I'll use a boolean `animating` / `canMove`. Set true at move start, false after animation timeout.

Let me define cell size. Board is 4x4. Let me pick CELL = 100px, board = 400x400, margin. Tile size ~ 90px with gap 10. Actually let me use a grid with CSS grid for the board background cells, and absolutely position tiles.

Let me set: board width 480px, 4 cells → cell 120px, tile 110px, gap 10. Let me compute: cell = 100, tile = 92, gap = 4. board = 4*100 = 400. Tile width 92, left offset = (100-92)/2 = 4. position: left = c*100 + 4, top = r*100 + 4.

Let me use transform translate: translate(c*100, r*100) then the tile is at offset 4 within. Or simpler: left/top = c*100 + 4.

Let me set CELL=110, gap such that board = 462. Eh, let me just pick numbers. I'll use a container with CSS grid for background, cell size 100px, tiles 92px positioned via translate with 4px offset.

Let me now also think about win detection: any tile value >= 2048 → win. Show win message overlay but allow continuing. Game over: no empty cells and no merges possible.

Let me check possible moves: for each adjacent pair (horizontal and vertical) if both equal → can merge; if any empty cell → can move (spawn could... actually a move is possible if sliding changes board). Simplest game-over check: try all 4 directions; if all moved===false → game over.

Let me now write everything. Let me also handle score: persist in a variable; New Game resets.

Let me also ensure arrow keys don't scroll: addEventListener keydown preventDefault for arrow keys.

Let me write the file now.

Let me write the full index.html:

```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>2048</title>
<style>
  * { box-sizing: border-box; user-select: none; }
  html,body { margin:0; padding:0; height:100%; font-family: 'Clear Sans','Helvetica Neue',sans-serif; background:#faf8ef; color:#777; overflow:hidden; }
  .wrapper { display:flex; flex-direction:column; align-items:center; height:100%; padding-top:30px; }
  .header { display:flex; align-items:center; justify-content:center; gap:20px; margin-bottom:20px; }
  .title { font-size:60px; font-weight:280; color:#777; letter-spacing:2px; }
  .score-panel { display:flex; gap:15px; align-items:center; }
  .score-box { background:#bbada0; border-radius:6px; padding:10px 20px; text-align:center; }
  .score-box .label { font-size:12px; color:#f9f6f2; text-transform:uppercase; }
  .score-box .value { font-size:28px; color:#f9f6f2; font-weight:bold; }
  .board { position:relative; width:448px; height:448px; background:#8eb0ad; border-radius:10px; padding:15px; display:block; }
  .grid { display:grid; grid-template-columns: repeat(4, 1fr); grid-template-rows: repeat(4,1fr); gap:6px; width:100%; height:100%; }
  .cell { background:#cbc2b3; border-radius:6px; width:100%; height:100%; }
  .tile { position:absolute; width:96px; height:96px; border-radius:6px; display:flex; align-items:center; justify-content:center; font-weight:bold; font-size:40px; color:#f9f6f2; transition: transform 0.18s ease, box-shadow 0.18s ease; }
  .tile.move { box-shadow: 0 0 20px rgba(0,0,0,0.3); }
  .tile.merge { animation: mergePop 0.24s ease; }
  @keyframes mergePop { 0% { transform: translate(-50%,-50%) scale(0); } 50% { transform: translate(-50%,-50%) scale(1.25); } 100% { transform: translate(-50%,-50%) scale(1); } }
  .tile.new { animation: appear 0.18s ease; }
  @keyframes appear { 0% { transform: translate(-50%,-50%) scale(0); opacity:0.4; } 100% { opacity:1; } }
  ...tile colors...
  .overlay { position:absolute; top:0;left:0;right:0;bottom:0; display:flex; align-items:center; justify-content:center; flex-direction:column; font-size:48px; color:#f9f6f2; text-shadow:0 0 20px rgba(0,0,0,0.5); z-index:2; pointer-events:none; }
  .overlay.hidden { display:none; }
  .button { background:#8f3a06; color:#f9f6f2; border:none; border-radius:6px; padding:12px 28px; font-size:20px; cursor:pointer; margin-top:10px; }
  .footer { margin-top:20px; font-size:14px; color:#777; }
</style>
</head>
<body>
<div class="wrapper">
  <div class="header">
    <div class="title">2048</div>
    <div class="score-panel">
      <div class="score-box"><div class="label">Score</div><div class="value" id="score">0</div></div>
      <button class="button" id="newGame">New Game</button>
    </div>
  </div>
  <div class="board" id="board">
    <div class="grid" id="grid"></div>
    <div class="overlay hidden" id="overlay"></div>
  </div>
  <div class="footer">Use arrow keys to slide tiles. Merge equal tiles to reach 2048.</div>
</div>
<script>
  const SIZE = 4;
  let tileIdCounter = 0;
  function tileId(){ return ++tileIdCounter; }
  
  let board; // 2D array of {value, id} or null
  let tiles; // Map<id, {value, r, c, el}>
  let score = 0;
  let canMove = true;
  
  const gridEl = document.getElementById('grid');
  const scoreEl = document.getElementById('score');
  const overlayEl = document.getElementById('overlay');
  const newGameBtn = document.getElementById('newGame');
  const boardEl = document.getElementById('board');
  
  const CELL = 100; // not used directly, tiles positioned absolutely
  
  // Actually let me size: board 462, grid has padding 15, cells ~ 105 each with 6 gap...
  // Let me set board width = 462, grid-area with 15 padding, gap 6. Cell = (462-15*2-6*3)/4 = (462-30-18)/4 = 414/4=103.5. Hmm not clean.
```

Let me reconsider sizing to keep it clean. Board width 448. Padding 15 each side → inner 418. Gap 6*3=18 → cell = (418-18)/4 = 400/4 = 100. Tile = 96 (with 4px margin/gap within cell? Actually gap 6 already separates cells). If tile width = 96 and cell = 100, offset within cell = 2. But gap handles spacing. Hmm, tiles are absolute positioned over the grid, so tile should fill cell area minus a small margin to show cell background. Tile 96 in a 100 cell → 2px margin each side. Position: left = c*100 + 15(board padding) + 2, top = r*100 + 15 + 2.

Let me just compute offsets in JS:
```
const BOARD_W = 448;
const PADDING = 15;
const GAP = 6;
const CELL = (BOARD_W - PADDING*2 - GAP*3) / 4; // = (448-30-18)/4 = 400/4 = 100
const TILE = CELL - 4; // 96, leave 2px each side for spacing (or rely on gap)
```
Position: left = PADDING + c*CELL + (CELL - TILE)/2 = 15 + c*100 + 2 = 17 + c*100.
top = PADDING + r*CELL + 2 = 17 + r*100.

Let me verify: c=3 → 17+300 = 317, +96 = 413 = 15 + 3*100 + 96 = 15+300+96=411, plus we need within board width 448: 411 ≤ 448-15=433 ✓ (leaves 22 on right? no). Hmm 413 vs board inner right edge 448-15=433. 413 to 433 = 20 gap. That's extra. Let me recompute: cell = 100, tile = 96, so within cell tile starts at offset 2 (cell*0.5*(100-96)). Position = PADDING + c*CELL + 2. For c=3: 15 + 300 + 2 = 317, tile extends to 317+96 = 413. Board width 448, right padding 15 → content area 15..433. 413 < 433, leaving 20px. That's because cell (4*100=400) + gap*3 (18) = 418, + padding 30 = 448. So grid content spans 15..433 (width 418). Cell c=3 spans 15+3*100=315..415 (with gap, last cell 315..415+6..). Tile at 317..413. Fine, fits.

OK let me not overthink; I'll compute in JS and set positions via transform translate for transitions.

Actually, using `left/top` with transition works fine for sliding. Let me use left/top.

Let me simplify and just hardcode reasonable CSS and compute positions in JS.

Let me write the render function:

```js
function cellOffset(r, c) {
  return { x: PADDING + c * CELL + (CELL - TILE)/2, y: PADDING + r * CELL + (CELL - TILE)/2 };
}
```

Render tiles from the `tiles` map: for each tile, set el.style.left/top. To animate, when moving, I set left/top and CSS transition slides. But for newly added elements I want appear animation. For merged I want mergePop. I'll add classes accordingly and set final position; the appear/mergePop animations use transform scale (translate to center?) — but if I also use left/top transitions, there may be conflict.

Let me unify: position tiles via left/top (no transform for position). Appear animation uses scale via transform on a child, or uses opacity. MergePop uses transform scale — but transform conflicts with... no, I'm not using transform for position (using left/top). So transform is free for animation. Good.

But mergePop with transform scale needs the element centered properly; scale transforms from center by default. Fine.

For appear: I'll set opacity and maybe scale. Let me use a wrapper or just animate opacity + a small scale. Since transform is free, appear: scale(0.5)->scale(1) + fade. But newly inserted tile is already at its final left/top, so scale animates in place. Good.

Let me make tiles position via left/top with transition. For moved tiles, the left/top change transitions. For new/merged tiles, I add a class to trigger appear/mergePop (which use transform). But if a tile both moves AND is new, the appear animation plus left/top transition both run — fine.

Hmm, but there's a subtlety: when I reposition an existing element to a new left/top, the CSS transition animates the move. Good. For the spawned tile and merged tile, I create the element already at final position and add an appear class; the appear is opacity/scale. Good.

Problem: when removing source tiles in a merge, I want them to disappear (fade out). I'll add a class and remove from DOM after transition.

Let me now also handle the animation flag and sequencing with setTimeout matching transition duration.

Let me define durations: slide transition 0.16s, appear/mergePop 0.2s, spawn fade 0.18s. I'll set animating=true, run animations, then after max duration (~220ms) allow moves and spawn.

Actually spawning should happen after the slide animation, within the same move cycle. Let me sequence:
1. User presses key (canMove true, not game over).
2. canMove = false.
3. Compute move result (board, score delta, merges, moved).
4. Apply score update (visually update score, maybe with a pop).
5. Render: animate existing tiles to new positions (slide), handle merges (pop in + fade out sources), spawn new tile (after slide, with appear).
6. After animations done: check win/game over. canMove = true.

For step 5 spawn timing: I'll spawn after a short delay (after slide). Let me spawn after 180ms.

Let me write the render carefully. I'll maintain `tiles` map. Each render step:

```js
function renderMove(result) {
  const {board: newBoard, merges, scoreDelta} = result;
  // update score
  score += scoreDelta;
  scoreEl.textContent = score;
  
  // 1. Handle merges: create merged tile elements, fade out sources
  const mergeAt = {}; // (r,c) -> merged tile
  for (const m of merges) {
    const key = m.r+','+m.c;
    // create merged tile element
    const t = {value: newBoard[m.r][m.c].value, id: newBoard[m.r][m.c].id, r: m.r, c: m.c, el: null};
    createTileEl(t, true); // merged
    mergeAt[key] = t;
    // fade out source elements
    for (const [sr, sc] of m.sources) {
      const st = getTileByIdAt(sr,sc);
      if (st && !tiles.has(st.id)) {} 
      // fade out st.el
      fadeOutAndRemove(st.el);
    }
  }
  ...
}
```

This is getting complicated because I need to coordinate ids and elements. Let me restructure the whole data model to be element-centric.

**Cleaner element-centric approach:**

Maintain:
- `board`: 2D array of numbers (values). This is the source of truth for logic, win, gameover, spawning.
- `tiles`: 2D array (same dims) where each cell is a tile object {value, el, id} or null, representing currently-rendered/animated tiles. This drives rendering.

Actually, let me maintain `board` (values) and `tileEls` grid. The animation: when moving, I know new board positions. For each cell, I determine the new tile. I animate by moving elements. For merges I need sources.

Let me just keep BOTH: board values for logic, and a grid of rendered tile objects with positions/ids. On each move:
1. Run move logic on board values → newBoard, scoreDelta, merges, moved.
2. If not moved: do nothing (canMove already false... actually canMove true, just no spawn, allow next move). Set canMove=true (never went false? I set false at start). Let me set canMove false only if moved, or always briefly. Let me always set canMove=false during animation even if no move, with tiny timeout. Actually if no move, no animation; just allow immediately. Let me handle: if !moved, skip animation, canMove stays true, no spawn.
3. If moved: 
   a. Update board = newBoard. Update score.
   b. Spawn: place new tile in newBoard (random empty cell). Update board.
   c. Animate: compute which tiles moved/merged/born by diffing old rendered grid vs new board.
   d. Perform animations; on finish canMove=true.

For the animation diff with merges, I'll use the merges info from the move logic (sources + target), and for non-merged moved tiles, I'll move their elements. But after spawn, the new tile is "born" (appear). 

Let me track rendered tiles by id in a map: `tileStore = Map<id, {value, r, c, el}>`. Board (values) + tileStore coordinated.

Move step with animation:
- oldBoard = board.clone(); oldStore = copy of tileStore (id→{r,c}).
- result = moveBoard(oldBoard, dir) → {board:newBoard, score, moved, merges}.
- if !moved: ignore (canMove not disabled, or re-enable). return.
- board = newBoard. score += result.score. update score display.
- Spawn: addNewTile() → picks empty cell, returns {r,c,v,id} ; board[r][c]=v; this id is "new".
- Now animate from oldStore positions to new board positions:
  - For each new tile (cell with value v): 
    - id = board tile id (I need board to store ids). 

OK I really should store ids in the board. Let me make `board` store tile objects {value, id}. Then:
- oldBoard: 2D of {value,id} or null.
- result (move logic on objects) produces newBoard with tile objects (reusing ids for non-merged, new ids for merged), merges info, moved.
- oldIds = new Set of ids in oldBoard.
- After move (before spawn), board = newBoard.
- Spawn: create {value,v,id:new} at random empty cell, board cell set.
- Animate:
  - For each new tile object in board:
    - If id in oldIds: it's a pre-existing tile (moved or stayed). Reposition el to (r,c) (slide transition). If it's a merged tile... wait merged tiles have NEW ids, so they're "new" not pre-existing. But merged tiles should "pop" not "slide". 
    - If id NOT in oldIds: it's new (merged birth or spawn). 
  - Distinguish merged-birth vs spawn: merged birth tiles have id from result.merges (new id, at a merge target). Spawn tile is the one added after. I'll tag the spawn tile with a flag.
  
So:
- moved & existing tiles (id in oldIds, not a merge target): slide el to new pos.
- merged tiles (new id, merge target): create element, pop-in at target. Fade out source elements (the old tiles whose ids are gone). The source elements: the two source tiles had ids that are no longer in board → they get faded out. But I also need them to visually "come together" — for simplicity, just fade them out and pop the merged tile. Acceptable.
  - But the two source tiles might also have been moving (sliding) to the merge point before merging. E.g., column sliding down with a merge at bottom: sources at rows 1,2, target row 2... they're adjacent, small move. If sources are far, just fading them out mid-flight is slightly odd but okay-ish. Hmm. Actually if I reposition existing source elements to their source cells (their old positions) and then fade them while popping merged at target, and the merged tile's sources were at e.g. (1,0) and (2,0) while target (2,0)... target overlaps source (2,0)! 

This overlap issue: the merged tile pops at target (r,c), but one source might already be at (r,c) or near. Let me instead animate sources moving to target. 

Given time, let me do THIS cleaner merge animation:
- For a merge at target T with sources S1, S2:
  - Move source S1's element to T (slide), and source S2's element to T (slide) — both slide to same target.
  - When they arrive (after transition), kill S2's element (fade out), set S1's element value to merged value and play mergePop. 
  - Reuse S1's element and id? But merged tile has new id. For game logic, the merged tile has a new id (so it won't merge again same move—though move logic already handled that). For rendering, reusing S1's element is fine visually. But I store by id; let me just reuse S1's element but update its value and give it the merged tile's identity in the store.

Hmm, id consistency: after the move, board has merged tile with new id M. The element at T should represent M. Let me, in the store, after animation, have T → {id: M, value: mergedValue, el: S1's el}. S1 and S2 ids are removed from store.

OK this is doable but intricate. Given 15 min, let me write it carefully but not over-engineer the source-slide-to-target; I'll just move both sources to target and pop. Actually that double-move to same target is fine visually (two tiles converge).

Let me reduce scope: I'll implement the merge as: both source elements slide to the target cell; upon arrival, one is removed and the surviving element's value updates to the merged value with a "pop" (scale up). This conveys merging well.

Let me code the animation with these steps and setTimeout to reset canMove.

Actually, let me reconsider total time. This is a lot. Let me write a solid, correct game with decent animation. I'll prioritize correctness (verified behaviors) over perfect animation.

Let me write the move logic on tile objects including id reuse and merge detection, then a render that:
- Slides all existing (non-new) tiles to new positions.
- Pops in merged tiles at their targets (and fades out the source tiles that disappeared).
- Fades in spawned tile.

For "fades out source tiles that disappeared": a source tile id that's not in new board and not used as a survivor → fade out. In my merge handling, both sources disappear (ids removed), merged tile is new id. So both source elements fade out, and the merged element pops in at target. Simple and clean (no convergence slide). I'll go with pop-in + fade-out. 

Let me finalize the plan and write code.

Data:
- `board`: SIZE×SIZE array of {value, id} or null.
- `tiles`: Map<id, {value,id,r,c,el}>.
- Move: moveBoard(board, dir) → {board, score, moved, merges}. merges: [{r,c, value, sources:[[r,c],[r,c]]}].
  - Actually I need merges to know target value and sources for animation. Let me have moveBoard return merges with target r,c and the merged tile object reference.

Let me write moveBoard operating on board of {value,id} (null for empty):

```js
function moveBoard(board, dir) {
  let totalScore = 0, anyMoved = false;
  const merges = [];
  const newBoard = board.map(r => r.slice());
  
  for (let i = 0; i < SIZE; i++) {
    // build line cells in front-to-back order
    const cells = [];
    if (dir==='left') for (let c=0;c<SIZE;c++) cells.push({tile: board[i][c], r:i, c});
    else if (dir==='right') for (let c=SIZE-1;c>=0;c--) cells.push({tile: board[i][c], r:i, c});
    else if (dir==='up') for (let r=0;r<SIZE;r++) cells.push({tile: board[r][i], r, c:i});
    else if (dir==='down') for (let r=SIZE-1;r>=0;r--) cells.push({tile: board[r][i], r, c:i});
    
    const {out, score, moved, lineMerges} = slideLineObjs(cells);
    totalScore += score;
    anyMoved = anyMoved || moved;
    merges.push(...lineMerges);
    for (let k = 0; k < out.length; k++) {
      const cell = out[k];
      if (cell.tile) newBoard[cell.r][cell.c] = cell.tile;
      else newBoard[cell.r][cell.c] = null;
    }
  }
  return {board: newBoard, score: totalScore, moved: anyMoved, merges};
}

function slideLineObjs(cells) {
  const compacted = cells.filter(x => x.tile !== null);
  const out = [];
  let score = 0;
  const lineMerges = [];
  let i = 0;
  while (i < compacted.length) {
    if (i < compacted.length-1 && compacted[i].tile.value === compacted[i+1].tile.value) {
      const mergedTile = {value: compacted[i].tile.value*2, id: tileId()};
      score += mergedTile.value;
      const targetPos = {r: cells[i].r, c: cells[i].c}; // front position = cells[i] pos
      out.push({tile: mergedTile, r: cells[i].r, c: cells[i].c});
      lineMerges.push({r: cells[i].r, c: cells[i].c, value: mergedTile.value,
                       sources: [compacted[i].r+','+compacted[i].c, compacted[i+1].r+','+compacted[i+1].c],
                       sourceTiles: [compacted[i].tile, compacted[i+1].tile]});
      i += 2;
    } else {
      out.push({tile: compacted[i].tile, r: cells[i].r, c: cells[i].c});
      i += 1;
    }
  }
  // pad to cells.length
  for (let k = out.length; k < cells.length; k++) out.push({tile: null});
  // assign front positions (out[k] → cells[k])
  for (let k = 0; k < out.length; k++) {
    out[k].r = cells[k].r; out[k].c = cells[k].c;
  }
  const moved = out.some((o,k) => (o.tile?o.tile.value:0) !== (cells[k].tile?cells[k].tile.value:0)) || out.some(o=>o.tile) && lineMerges.length>0;
  // simpler moved: compare positions/ids
  const movedCheck = out.some((o,k) => {
    const ob = cells[k].tile, nb = o.tile;
    if (!ob && !nb) return false;
    if (!ob || !nb) return true;
    return ob.id !== nb.id || ob.value !== nb.value;
  });
  return {out, score, moved: movedCheck, lineMerges};
}
```

Wait, the `moved` detection by id: out[k].tile for a non-merged tile reuses compacted[i].tile (same object, same id) but now at position cells[k] (front). cells[k].tile is the original tile at that front position. If a tile moved, out[frontPos].tile.id != cells[frontPos].tile.id → moved. If no tile moved (stable), ids match → not moved. If a merge happened, the merged tile has a new id ≠ cells[k].tile.id (which was either null or a same-val tile) → moved. Good. But careful: if front position had no tile and a tile moves in, cells[k].tile=null, out[k].tile=id → moved. Good. 

Edge: slideLineObjs out positions: I assign out[k].r/.c = cells[k].r/.c. But in the merge branch I set out tile r/c = cells[i].r/.c (front) then reassign in loop. Fine. Also out.length might be < cells.length when padded; I push {tile:null} without r/c then assign in loop. Let me ensure loop covers all. Yes loop k from 0 to out.length-1 assigns r/c. For padded nulls, out[k].r/.c set to cells[k]. Good. But null entries: `out[k].r` accessed — fine.

Hmm wait, in compacted I keep cells with tile. out built with r/c = cells[i] initially (source front... actually cells[i] is the i-th compacted cell, which is at the front already because cells is front-to-back and compacted preserves order). Since compacted = cells.filter non-null, compacted[i] corresponds to some cells[j]. The front position for output k is cells[k]. So I correctly reassign out[k].r/.c = cells[k]. Good.

movedCheck uses cells[k].tile vs out[k].tile. Good.

Now `moved` overall = any line moved. But also if merges occurred without net position change? A merge always changes value (doubles) so ids differ → moved true. Good. Empty move (no tile can move) → all lines not moved → moved false. 

Now animation render. After computing result (newBoard with tile objects, merges), I:
- Set board = result.board.
- Update score.
- Spawn new tile.
- Animate:
  - For each old tile id (from previous tiles map) not in new board → fade out (these are consumed/merged or just vacated).
    - But vacated tiles that just slid away should be REMOVED (not faded) since they reappear at new position as the SAME id (slid tiles reuse id). So a slid tile keeps its id and element; I just reposition. Its old cell empties but the element moved, so no removal needed at old cell. 
    - Faded-out ids: tiles whose id is NOT in new board. These are: (a) merged source tiles (2 per merge), (b) tiles that... no, slid tiles keep id. So faded ids = merged sources. Also spawn is new id (not in old).
  - For each tile id in new board:
    - If id in old tiles (pre-existing, i.e., a slid tile) → reposition its el to new (r,c) with slide.
      - But merged tiles have NEW ids → not in old → handled as "new/pop".
    - If id not in old tiles → new tile (merged or spawn) → create element, pop-in at (r,c) with appear or mergePop.
  - merged tiles: pop (mergePop) at target. spawn: appear (fade/scale).
  - source tiles (merged sources): their ids not in new board → fade out.

But wait: a moved-but-not-merged tile keeps its id and is repositioned. Good, slides. A merged tile: new id, sources fade out, merged tile pops. A spawned tile: new id, pops (appear). 

I need to differentiate merged-pop vs spawn-appear for the animation class. merges list gives merged tile positions/ids. spawn gives spawn position. Let me tag: new tiles whose id is the merged tile's id → mergePop; the spawn tile → appear. 

Let me track spawned tile id. And merged tile ids (from result.merges, each merge has the merged value and target but not id directly—actually out[k].tile.id for merged). Let me have moveBoard return merges with the tile id. Let me add `id` to merge objects.

Let me also reconsider: after move, the new board tile objects for non-merged tiles are the SAME objects (same ids) carried from old board (reused). Good. Merged tiles are new objects (new ids). Spawn is new.

So:
- oldIds = set of ids in old tiles map.
- newIds = set of ids in new board.
- For id in newIds ∩ oldIds: reposition el (slide).
- For id in newIds \ oldIds: new tile. If id is a merged id → mergePop; if id is spawn id → appear.
- For id in oldIds \ newIds: fade out el (these are merged sources).

But merged sources: are their ids necessarily in oldIds? Yes (they existed before). And removed from new board → in oldIds \ newIds → fade out. Good. But these source elements are at their source positions; I fade them out where they are (or I could slide them to target first). For simplicity, fade out in place. Slight visual gap but acceptable. Actually, to make merges look better, let me slide source elements to the target then pop. But that requires knowing which source goes to which target and timing. Given budget, fade-out-in-place + pop-in at target is acceptable and clearly communicates merge.

Hmm, but fade out in place while a new tile pops at target (which might be a different cell) — fine.

Actually, wait: there's overlap risk if target cell is where a source was. E.g., merge sources at (1,0),(2,0), target (2,0)? No—let me check. For left merge in a row, sources are adjacent, target = front source position. E.g., row [2,2,0,0] left: compacted sources (0,0)&(1,0)... wait front position cells[0] = (r,0). Sources = (r,0) and (r,1). Target = cells[0] = (r,0). So source at (r,0) is also the target! The merged tile pops at (r,0) while source at (r,0) fades out — same cell, pop over fade. The other source at (r,1) fades out. OK overlap of fade and pop at same cell — pop animates on top, fine.

But the source at (r,0) had value 2, target merged value 4 pops at (r,0). The fade-out of the 2 and pop-in of 4 at same spot is okay.

Alright. Let me also make sure non-merge moved tiles slide: a tile at (r,1) moving to (r,0) (left slide) keeps id, reposition el from (r,1) to (r,0) via left/top transition. Good. No fade.

Let me now also handle z-index so popping merged tile is on top, and sliding tiles don't get hidden. I'll set z-index by value or arbitrary high.

Let me write createTileEl:
```js
function createTileEl(tileObj, appearClass) {
  const el = document.createElement('div');
  el.className = 'tile ' + tileValueClass(tileObj.value) + (appearClass ? ' ' + appearClass : ' new');
  el.textContent = tileObj.value;
  el.dataset.id = tileObj.id;
  const off = cellOffset(tileObj.r, tileObj.c);
  el.style.left = off.x + 'px';
  el.style.top = off.y + 'px';
  boardEl.appendChild(el);
  tileObj.el = el;
}
```

Wait, `new` appears class for spawned; `merge` for merged. Let me pass a string: 'new' or 'merge'. Default slide uses no appear class but I set positions via left/top transition. For new/merged, I set final position and add the class.

Hmm, for a NEW tile (spawn or merge) I create the element at final position and add 'new'/'merge' class for pop-in. The element isn't sliding (it's already at target). Good.

For slid tiles (existing id), I update el.style.left/top to new position; CSS transition slides. But on first render (initial board) there's no transition desired? It's fine, or I set transition none for initial. I'll just set positions; initial board render no animation is fine, or I can add a tiny appear. Let me not animate initial placement (set immediately). Actually I'll create initial tiles with 'new' appear for nice effect. Optional. Let me keep simple: initial tiles placed immediately with class based on value, no 'new'. Or add 'new' for all—looks nice. I'll add a short appear for initial. Eh, let me add 'new' for spawns and initial, 'merge' for merges.

Let me now think about the slide transition for existing tiles. When I set new left/top on an element that already has a position, transition animates. Good. But I must SET the position AFTER adding to DOM / after a frame to ensure transition applies. Since elements already exist, just setting style triggers transition. Good. For the transition to actually slide, the element must move from old to new position in one step (no intermediate commit). I'll set all positions in a loop; browser batches. Fine.

Potential issue: I'm reusing the SAME element for a tile that moved. The element's id stays the same; I reposition. Good.

Now coordinate the animation end and canMove:
- Set canMove=false at move start.
- After scheduling animations, setTimeout(reset canMove, 350ms) (covers slide+pop). Then handle win/gameover/next spawn already done.

Spawn timing: spawn happens immediately in board (logic) but the element appears with 'new' animation. I could delay spawn element creation until after slide. But simpler: include spawn in the same render; the spawn element appears (scale up) while others slide. Slightly overlapped but fine. Actually a cleaner 2048 feel: slide completes, then new tile appears. Let me delay the spawn element creation by ~200ms. I'll create spawn el after a timeout.

Let me structure renderMove:
```js
function renderMove(result, spawnCell) {
  // result: {board, merges, score, moved}
  // update score
  score += result.score;
  scoreEl.textContent = score;
  
  // determine new ids set
  const newIds = new Set();
  for (let r=0;r<SIZE;r++) for (let c=0;c<SIZE;c++){ const t=board[r][c]; if(t) newIds.add(t.id); }
  
  const mergedIds = new Set(result.merges.map(m=>m.id));
  
  // 1. fade out old tiles not in new board (merged sources)
  for (const [id, t] of tiles) {
    if (!newIds.has(id)) {
      fadeOutAndRemove(t.el);
      tiles.delete(id);
    }
  }
  
  // 2. handle new board tiles
  for (let r=0;r<SIZE;r++) for (let c=0;c<SIZE;c++){
    const cell = board[r][c];
    if (!cell) continue;
    if (tiles.has(cell.id)) {
      // existing: reposition (slide)
      const t = tiles.get(cell.id);
      t.value = cell.value; t.r = r; t.c = c;
      // update text maybe (value unchanged for slid)
      const off = cellOffset(r,c);
      t.el.style.left = off.x+'px'; t.el.style.top = off.y+'px';
    } else {
      // new tile: merged or spawn
      // create element
      const el = document.createElement('div');
      const cls = mergedIds.has(cell.id) ? 'tile merge-pop ' + tileClass(cell.value) : 'tile new ' + tileClass(cell.value);
      el.className = cls;
      el.textContent = cell.value;
      const off = cellOffset(r,c);
      el.style.left = off.x+'px'; el.style.top = off.y+'px';
      el.dataset.id = cell.id;
      boardEl.appendChild(el);
      tiles.set(cell.id, {value:cell.value, r, c, el});
    }
  }
  
  // spawn element (if not created as part of board above? spawn tile was added to board, so it's created above as 'new')
  ...
}
```

Wait, I add spawn to board BEFORE render, so the spawn tile (new id) is created in step 2 as 'new' (appear). That's fine—spawn appears. But I wanted spawn AFTER slide. With this, spawn appears simultaneously with slide. Acceptable. But the spawn cell is empty in the board only after I add it; the element appears at spawn cell. Fine.

Hmm, but there's an issue: the spawn tile id is new, so it's created via the 'new' appear. Good.

But I also need to handle that during the slide, source elements for merges fade out. In step 1 I fade out all ids not in new board. Merged source ids are not in new board → fade out. Good. But they fade out IN PLACE (at their old positions), while the merged tile pops at the merge target. For a left-merge where target == source cell (r,0), the source at (r,0) fades while merged pops at (r,0) — same location, the new element (popped, z-index high) covers. OK.

Let me set z-index: merged/new tiles higher. I'll set inline z-index by value order or just give new/merged tiles z-index 10, slid tiles normal. Eh, let me give all tiles z-index = value-based or just high enough. I'll set slid tiles z-index 1 and new/merged z-index 2. Actually fading source at same cell as pop: pop's element appended later (higher in DOM) → on top. Fine.

One concern: fadeOutAndRemove on a source element that is ALSO being repositioned? No—sources are removed (not in new board), slid tiles keep id. No conflict.

Another concern: an element that is both a slid tile (id in new board) — reposition, no fade. Good.

Now `tileClass(value)` returns color class per value. I'll define CSS classes for values 2..2048+ with distinct colors.

Let me also handle the win overlay: when any tile value >= 2048, show "2048!" overlay, allow continue. Show once? I'll show overlay when win achieved, and a "Keep Going" button or click to dismiss, but allow continuing via keys. Let me show overlay with "You Win!" and a note, plus keep playing. I'll set a flag `won` and not re-trigger. Game over: no valid moves → show overlay "Game Over" with New Game button.

Let me make the overlay block keys when shown? For win, allow continue (keys still work, maybe dismiss overlay on any move). Let me: on win, show overlay; on next move, hide overlay and continue. On game over, show overlay and disable moves until New Game.

Implementation: overlayEl shows text + button. For win: text "2 0 4 8!" and small "Keep going!" ; clicking or pressing key dismisses. For gameover: "Game Over". 

Let me handle: after move+spawn, check win (board has 2048) → if !gameOver && !won: show win overlay, set won=true. If game over (no moves): show game over overlay, canMove=false (already). Check gameOver by trying all 4 dirs: if all moved false → gameover.

Let me write checkGameOver(board): for each dir, moveBoard(board,dir).moved; if any true → not game over. But moveBoard creates new boards; I just check moved. Need a version that doesn't mutate board and doesn't spawn. My moveBoard takes board and returns new board without mutating (it copies). Good, use it for check.

Now input handling: keydown listener on document; if !canMove return; preventDefault; map arrow to dir; if move changes board, proceed. 

Also touch swipe support? Not required. I'll add basic keyboard. Maybe also arrow buttons? Required: arrow keys. I'll do keyboard primarily. Could add on-screen arrows for robustness (the verification "Play a short sequence of arrow-key moves"). Browser simulation: I'll test with keydown events via JS or via the browser tool's key. Let me add keyboard + maybe click buttons as fallback.

Let me also ensure preventDefault on arrow keys to stop scroll: addEventListener('keydown', e => { if (['ArrowUp','ArrowDown','ArrowLeft','ArrowRight'].includes(e.key)) { e.preventDefault(); ... } }, {passive:false}). But passive:false needed for preventDefault on touch/scroll; for keydown it's fine. Actually for keydown, preventDefault works without passive concern generally, but to be safe add {passive:false, capture:true}? Let me add on window keydown with preventDefault for arrows. Also add touchAction none on body to prevent scrolling/pinch.

The page: body overflow hidden; arrow keys could still scroll if focusable elements. I'll preventDefault on keydown for arrows. Good.

Now the "animate movement and merging" CSS: .tile { transition: left 0.16s ease, top 0.16s ease; } for slide. .new { animation: appear 0.18s ease; } .merge-pop { animation: mergePop 0.2s ease; }. appear: scale from 0.5 + fade. mergePop: scale from 1.25 to 1 quickly.

Wait, appear uses transform scale, but slide uses left/top (no transform). For new tiles I use 'new' (scale appear). For merged 'merge-pop'. For slid tiles, only left/top transition. Good, no conflict.

But there's subtlety: when a slid tile (existing element) gets repositioned, its transform is none (scale 1). If previously it had no animation class, fine. If a tile was 'new' (spawn) and then in a future move it slides, I should remove 'new' class to avoid re-transform issues. Let me ensure when repositioning existing tiles, I remove 'new'/'merge-pop' classes and ensure value text updated. Let me set t.el.className properly each time based on value, keeping no animation class for plain moves. Actually for a tile that just slides (not new), I want only the transition. So set className = 'tile ' + tileClass(value) and let transition handle left/top. Good.

For created new/merged tiles, className includes 'new' or 'merge-pop' plus value class.

Let me make sure value text updates when a tile's value changes (only merged, but merged creates new element). Slid tiles keep value. Fine.

Now the score pop: when score increases, I could animate the score number. Optional. I'll add a quick color flash. Keep simple: just update text; maybe add class 'scored' with animation. I'll skip for time, or do a quick flash. Let me add a simple flash.

Let me also add a best-score? Not required. Skip. But nice. I'll add localStorage best score? The task says reset score on New Game. Best score is separate; not required. Skip to keep simple. Actually let me include best score persistence as a nice touch—it's quick. Hmm, but "reset board and score" — score must reset; best can persist. I'll include best with localStorage. Optional, low risk. I'll add it.

Let me now also double check spawn probability: 2 with 90%, 4 with 10%. `Math.random() < 0.1 ? 4 : 2`.

Spawn in random empty cell: collect empty cells, pick random, set value.

Initial: two tiles, each 2 (90%) or 4. I'll just spawn two via the spawn function (which places 2/4). Start score 0.

Let me now also ensure "Do not spawn after an unchanged move": if !moved, don't spawn, don't animate, canMove stays true. Good.

Now, the spawn should occur after the move animation? The task: "After a move that changes the board, spawn exactly one tile in an empty cell". The spawn tile could appear with the move or after. I'll spawn as part of the move (element appears). Fine.

Edge: if moved but board is full after move (no empty) → can't spawn. In real 2048, spawn only if empty cell exists. The task says "spawn exactly one tile in an empty cell; ... Do not spawn after an unchanged move." Implies spawn only if empty cell. If board full after a merge move, no spawn. I'll only spawn if there's an empty cell. (Rare.)

Now let me also handle win then continue: after win, subsequent moves still work and merges can make tiles > 2048. Win overlay shown once. If game over reached after win, show game over.

Let me code checkWin: board has any tile value >= 2048 → won (trigger once). 

Let me now write the tile color classes. I'll define for values 2,4,8,16,32,64,128,256,512,1024,2048,4096,8192. Colors from original 2048 theme. Let me include a reasonable set.

Now let me write the full file. I'll be careful and complete.

Let me also make the board cell visually show the grid: .grid with gap background cells. Tiles absolute positioned over it.

Let me write it now.

One more: the `cellOffset` must match the grid cell positions. Board padding 15, gap 6, cell = 100, tile = 92 (so 4px total margin, 2 each side? tile 92 in 100 cell leaves 8, split 4 each side for centering). Let me set TILE=92, offset within cell = (100-92)/2 = 4. Position left = 15 + c*100 + 4 = 19 + c*100. Hmm wait cell width is 100 including gap? In CSS grid with gap 6, the cells are 100 wide but 6 gap between. The cell's x-origin = 15 + c*100 (since each column occupies 100 including half-gaps). Actually with gap, column c starts at 15 + c*(100) ... no. Grid tracks: total = 15 (pad) + 4*100 + 3*6 (gaps) + 15 = 448? 15+400+18+15 = 448. ✓. Column c's cell area (the content box) starts at x = 15 + c*100 + c*6? No: tracks: col0 content starts at 15, width 100, then gap 6, col1 starts at 15+100+6=121, col2 at 15+100+6+100+6=227, col3 at 15+100+6+100+6+100+6=333. So col c start = 15 + c*100 + c*6 = 15 + c*106. Hmm that means each column advances by 106 (100+6). For c=3: 15+3*106=15+318=333, +100=433, +15 pad=448 ✓.

So cell start x(c) = 15 + c*106. Tile of width 92 centered: left = x(c) + (100-92)/2 = x(c) + 4. For c=3: 333+4=337, +92=429, +15=444... within 448 ✓ (board outer). Good.

Wait, but is the cell "width" exactly 100 in CSS grid? With grid-template-columns: repeat(4, 1fr) and gap 6, the fr units distribute. If board inner width = 448-30(pad) = 418, minus gaps 3*6=18 → 400 for 4 columns → 100 each. So yes each column 100, gap 6. Column start x(c) = 15 (pad) + c*(100+6) = 15 + 106c. ✓.

So cellOffset(c) x = 15 + c*106 + 4 = 19 + c*106. y similar = 19 + r*106. TILE=92.

Let me define:
```
const PAD=15, GAP=6, CELL=100, TILE=92;
const step = CELL+GAP; // 106
function cellOffset(r,c){ return {x: PAD + c*step + (CELL-TILE)/2, y: PAD + r*step + (CELL-TILE)/2}; }
```
(CELL-TILE)/2 = 4. So x = 15 + 106c + 4 = 19+106c. y=19+106r. Tile 92. c=3: x=19+318=337, width end 429, +15 pad right... board outer 448, so 429 ≤ 448 ✓. Good. c=0: x=19.

Wait 19+106*3 = 19+318=337; 337+92=429; board width 448; 429<448 ✓. But the grid's last cell content starts at 333 (15+318). Tile centered at 333+4=337. ✓.

Now board width: outer = 448. Let me set .board { width:448px; height:448px; padding:15px; } and .grid { width:100%; height:100%; display:grid; grid-template-columns:repeat(4,1fr); grid-auto-rows:1fr; gap:6px; } The grid inside padded board: inner = 418. Columns 100 each + gaps. ✓. Tiles absolute positioned relative to .board (position:relative). boardEl relative. cellOffset uses PAD=15 matching board padding. ✓.

But .board padding:15 and .grid width:100% → grid area = 418. Good.

Let me make CELL computed from CSS to avoid mismatch? Risk of mismatch if my CSS numbers differ. I'll hardcode consistently. Let me set BOARD_W=448, PAD=15, GAP=6, CELL=100. In CSS .board width:448 height:448. .grid gap:6. Tiles width/height:92. Position via JS using these constants. Consistent.

Actually, if I set tile size 92 and cell 100, the tile is 92 and there's 4px margin (showing cell bg) + gap 6 between cells. Actually gap is between cells; tile 92 centered in 100 cell leaves 4px each side; the gap 6 is additional between cells. So visual gap between adjacent tiles = 4 (right margin) + 6 (gap) + 4 (left margin) = 14px of cell bg visible. Fine, looks like the original 2048 spacing.

Let me write the file now. I'll include tile color classes.

Let me write CSS for tiles:
```
.tile { position:absolute; width:92px; height:92px; border-radius:6px; display:flex; align-items:center; justify-content:center; font-weight:bold; font-size:40px; color:#f9f6f2; transition:left .16s ease, top .16s ease; }
.tile.v2 { background:#eee4da; color:#776e65; }
.tile.v4 { background:#ede0c8; color:#776e65; }
.tile.v8 { background:#f2b179; color:#f9f6f2; }
.tile.v16{background:#f59563;color:#f9f6f2;}
.tile.v32{background:#f67c5e;color:#f9f6f2;}
.tile.v64{background:#f65e3b;color:#f9f6f2;}
.tile.v128{background:#edcf71;color:#f9f6f2;font-size:36px;}
.tile.v256{background:#edcc61;color:#f9f6f2;font-size:36px;}
.tile.v512{background:#edc850;color:#f9f6f2;font-size:36px;}
.tile.v1024{background:#edc53f;color:#f9f6f2;font-size:28px;}
.tile.v2048{background:#edc22e;color:#f9f6f2;font-size:28px;}
.tile.v4096{background:#3c3a32;color:#f9f6f2;font-size:28px;}
.tile.v8192{background:#3c3a32;color:#f9f6f2;font-size:28px;}
.tile.new { animation: appear .18s ease; }
.tile.merge-pop { animation: mergePop .22s ease; }
@keyframes appear { 0%{ transform:scale(.3); opacity:.3; } 100%{ transform:scale(1); opacity:1; } }
@keyframes mergePop { 0%{ transform:scale(1.5); opacity:.9; } 100%{ transform:scale(1); opacity:1; } }
```

Appear uses transform scale; but if a tile is 'new' and also needs to be positioned before animation, transform scale animates from center. Good. But note: I set left/top for position immediately; appear only scales. Good.

Wait, for newly created tiles I set left/top immediately and add 'new' class → scale animates from .3 to 1, opacity. Good.

Now, an issue: when I reposition a slid tile that previously had 'new' or 'merge-pop' class lingering, I reset className. Let me always set className = 'tile v'+value for slid tiles (no animation classes). Good.

Let me also ensure tile font size adjusts for big numbers; using classes. For values > 2048, use v8192 styling. I'll default unknown big values to a dark color. Let me add a default for unknown (e.g., .tile[v-class-default]). I'll just compute class = 'v' + value if known else 'v8192' style (dark). I'll include classes up to 8192 and fallback to dark.

Let me now also handle the score element update and best score.

Let me write the code. I'll also add a simple "score pop" when scoring.

Let me write full index.html now.

For the win/gameover overlay, I'll make it cover the board with text and a restart button. For win, allow continuing: after win, pressing a key dismisses overlay and continues (or I keep overlay but allow keys to pass). Let me: show overlay for win containing "2 0 4 8!" and text "Keep playing" — click or key dismisses. For gameover, overlay "Game Over" with New Game button. I'll make overlay pointer-events none for win (so keys still pass) — but then it just shows. For gameover, disable moves (canMove false) and overlay has button.

Let me implement:
- showWin(): overlay text "2 0 4 8!", subtitle "Keep going!", show. won=true. (overlay semi-transparent over board). Keys still work; on next move, the board changes and I can hide overlay automatically. Let me hide win overlay on any successful move. 
- showGameOver(): overlay "Game Over". won irrelevant. canMove stays false. Show New Game button.

Let me handle overlay: a single overlayEl; set innerHTML/textContent and visibility. For win, show with "Keep playing!" and hide on next move. For gameover, show button.

Let me code. I'll keep overlay always covering board when shown.

Let me also make New Game button and overlay button call resetGame.

resetGame: clear board, tiles, score=0, won=false, gameOver=false, remove all tile elements, hide overlay, init board with 2 tiles.

Let me write init: create board (all null tile objects? I'll use board values as tile objects {value,id} or null). Place two random tiles. Render.

Let me store board as 2D of {value,id}|null.

initTile(r,c,v): board[r][c]={value:v,id:tileId()}; create el.

spawnTile(): pick random empty cell, set board cell, return {r,c,v,id}. In renderMove I create the el for it (as 'new').

Let me restructure renderMove to handle everything including spawn el creation.

Let me write the code fully now. I'll be careful.

Let me write move handler:
```js
function handleMove(dir) {
  if (!canMove || gameOver) return;
  const oldIds = collectIds();
  const result = moveBoard(board, dir);
  if (!result.moved) {
    // no change; allow next move (no animation)
    return;
  }
  canMove = false;
  board = result.board;
  score += result.score;
  updateScore();
  // spawn
  let spawned = null;
  if (hasEmpty()) spawned = spawnTile(); // adds tile object to board
  animateMove(result, spawned);
  // win/gameover checks done after animation
}
```

animateMove:
```js
function animateMove(result, spawned) {
  const mergedIds = new Set(result.merges.map(m=>m.id));
  const newIds = collectIds();
  // fade out old tiles not in new board
  for (const [id,t] of tiles) {
    if (!newIds.has(id)) {
      fadeOutAndRemove(t.el);
      tiles.delete(id);
    }
  }
  // update/create tiles
  for (let r=0;r<SIZE;r++) for (let c=0;c<SIZE;c++){
    const cell = board[r][c];
    if (!cell) continue;
    if (tiles.has(cell.id)) {
      const t = tiles.get(cell.id);
      t.value = cell.value; t.r=r; t.c=c;
      // reset classes (no animation) — set className
      t.el.className = 'tile ' + tileClass(cell.value);
      t.el.textContent = cell.value;
      const off = cellOffset(r,c);
      t.el.style.left = off.x+'px'; t.el.style.top = off.y+'px';
    } else {
      const el = document.createElement('div');
      const isMerged = mergedIds.has(cell.id);
      const cls = 'tile ' + tileClass(cell.value) + (isMerged ? ' merge-pop' : (spawned && spawned.id===cell.id ? ' new' : ''));
      // Actually spawned tile id is new and in newIds; merged ids separate. For spawn, class 'new'.
      el.className = cls;
      el.textContent = cell.value;
      el.dataset.id = cell.id;
      const off = cellOffset(r,c);
      el.style.left = off.x+'px'; el.style.top = off.y+'px';
      boardEl.appendChild(el);
      tiles.set(cell.id, {value:cell.value, r, c, el});
    }
  }
  // after animations, reset canMove and check win/gameover
  setTimeout(() => {
    canMove = true;
    checkGameState();
  }, 300);
}
```

Wait, the spawned tile: spawned.id is new, not in oldIds, so it's created in the else branch with class based on spawned.id===cell.id → 'new'. Good. But I also need merged tiles (new id, merge target) created with 'merge-pop'. mergedIds.has(cell.id) → true → merge-pop. Good. But note spawned tile id is also new (not in oldIds) and not merged → 'new'. Good.

But there's an ordering issue: a merged tile's id is in newIds (it's in board). tiles doesn't have it → else branch → mergedIds has it → 'merge-pop'. Good.

However: the merged tile is at the target cell which in the new board holds the merged tile object (value doubled, new id). Its el is created fresh at target with merge-pop. The source elements faded out. Good.

But the source elements: they are at their OLD positions and fade out. But during the move, non-source slid tiles slide to new positions. The source tiles themselves—do they slide? In a merge, both sources are consumed (not in new board as themselves). So they don't slide; they just fade out. But visually they should maybe slide to merge point. For 'left' merge in a row, sources are at cells (r,0) and (r,1); target (r,0). Source at (r,0) is at target already; source at (r,1) fades in place (one cell left of target). Eh, slightly off but acceptable.

Actually, wait: consider a vertical merge (down). Column [2(bottom? no, top first)] Let me think column [0,2,2,4] down → targets row3? Earlier example column [0,2,2,4] down → [0,0,4,4] with merge at row2? Let me recompute with new logic. Sources the two 2's at rows1,2. They merge into 4. Target = front position in down line = cells[0] of down line = the bottom cell (r=3,c). Wait down line front = bottom row (r=3). So target = (r=3, c). Merge at bottom. Result column [0,0,0,8]? 

Hold on, let me recompute column [0,2,2,4] (rows 0..3: row0=0,row1=2,row2=2,row3=4) moving down:
- down line cells front-to-back = bottom first: cells = [(r3,c), (r2,c), (r1,c), (r0,c)] = values [4,2,2,0].
- compacted (non-null/val!=0): [(3,4),(2,2),(1,2)] → values [4,2,2].
- i=0: 4≠2 → out[(r3,c)]=4. i=1.
- i=1: 2==2 → merge → merged 4 at front position cells[1] = (r2,c). sources (r2,c)&(r1,c). i=3.
- out: [4 (r3), 4 (r2) from target (r2,c)], pad 2 nulls.
- new board column: r3=4, r2=4, r1=0, r0=0 → [0,0,4,4]. ✓ Merge at (r2,c), sources (r2,c) and (r1,c). 

So target (r2,c) = one of the sources (r2 had value 2, the bottom-ish 2). The merged 4 pops at (r2,c); source at (r2,c) fades (same spot—pop over fade), source at (r1,c) fades in place. Meanwhile the 4 that was at (r3,c) stays (id reused) → no slide. Visually the bottom 4 stays, two 2's at rows1,2: one fades at row2 (merge pop there), one fades at row1. Acceptable.

Hmm, ideally the two 2's would slide down and merge at row3? No—the rule: 4 at row3 stays; the two 2's (rows1,2) merge into a 4 at the lowest available = row2 (just above the 4). So [0,0,4,4]. Correct 2048 behavior. Good.

My animation: source 2 at row2 fades as merged 4 pops at row2; source 2 at row1 fades. The existing 4 at row3 stays put. This is fine.

OK. Let me also make sure `moveBoard` doesn't mutate the input board (it copies). In handleMove I do `board = result.board`. Good. But I call moveBoard on current board; result.board is a new array with new tile objects for merged and reused objects for non-merged. So old board objects: the non-merged tiles are the SAME objects (reused) now in new board. The `tiles` map stores by id referencing el; the tile object {value,id} identity—I store el and value in tiles map, not the board object. So reuse is fine.

Wait, in moveBoard for non-merged, I push `tile: compacted[i].tile` (same object). And out[k].tile = that object. newBoard[cell.r][cell.c] = that object. So the object is shared. Good. Its id unchanged. Good.

Now `collectIds` reads board ids. Good.

Edge: after a move with merge, a non-merged tile might move to a new cell but keep id; tiles.has(id) true → reposition. Good. Its old cell empties (board old cell null) → no element there (element moved). Good. No fade.

But what about a tile that moves from (r,1) to (r,0): element repositioned from x(1) to x(0), slides. Old cell (r,1) now null → correct (element moved). Good.

Now fade out: ids in oldIds not in newIds = merged source ids only (since slid tiles keep ids). Good. But consider a tile that moves off-screen? All tiles stay on board (moved to another cell) → keep id. Only merged sources disappear. Good.

Wait, also: when a tile doesn't move but the board changed due to other tiles—its id still in newIds → reposition (no actual position change, transition 0). Fine.

Now, there's a subtle bug: after moveBoard, for a non-merged tile that STAYED in place (id same, position same), I still set left/top (same) → no visual. Fine. But I reset className to 'tile vX' removing any 'new' class — if it was a spawn from previous turn, 'new' class removed. Good (spawn class shouldn't persist).

Now canMove & animation timing: I set canMove=false, animate, setTimeout 300ms to re-enable. During this, keydown ignored. Good. prevent scroll on arrows still.

But what if moved but no animation needed (e.g., a merge where everything in place)? Still 300ms lockout—fine, brief.

Let me reduce lockout to 250ms.

checkGameState:
```js
function checkGameState(){
  if (!won && boardHas(2048)) { won=true; showWin(); }
  if (isGameOver()) { gameOver=true; showGameOver(); }
}
function boardHas(v){ for rows... if tile.value===v return true; also >= for win }
```
Actually win is any tile >= 2048. boardHasMax check.

isGameOver: no empty and no adjacent equal (horiz & vert) → true. Equivalent: all 4 directions moved===false. Let me compute by trying dirs on current board (copy, no spawn). 
```js
function isGameOver(){
  if (hasEmpty()) return false;
  for (const dir of ['left','right','up','down']) {
    if (moveBoard(board, dir).moved) return false;
  }
  return true;
}
```
moveBoard doesn't mutate board (copies). Good. But it creates tile objects? No—moveBoard on board of tile objects: for non-merged it reuses objects, for merged creates new ones. It returns a new board; I don't use it. Creates some objects but fine (garbage). Actually it won't create merged objects since I only check `.moved` (slideLineObjs still creates merged tile objects during computation but they're discarded). Slight waste but fine for 4x4.

Hmm, but creating new ids (tileId) during isGameOver check would increment counter unnecessarily—fine.

Actually wait, moveBoard calls slideLineObjs which on a merge creates merged tile objects with new ids and pushes to lineMerges and out. These are discarded. No harm. But it does mutate? moveBoard copies board into newBoard and fills newBoard with out tiles. It returns newBoard. I ignore it. The original board untouched (copies). But the non-merged tile objects are shared references; moved check fine. Good.

Let me make sure moveBoard doesn't accidentally mutate input: newBoard = board.map(r => r.slice()) — shallow copy of rows (slices the inner arrays, but tile objects shared). Then writes newBoard[cell.r][cell.c] = cell.tile. Doesn't touch board[r][c]. Good. Input board unchanged.

Now showWin/showGameOver: set overlay. For win, overlay shows "2 0 4 8!" and "Keep playing (arrows still work)". I'll hide win overlay when next move happens (in handleMove, if won, hide overlay). Or keep showing. The task: "Show ... a win message on reaching 2048 ... allow continuing after a win." So keep showing win message but allow continuing. I'll keep overlay visible but allow keys; next successful move hides it. Let me hide on next move.

Actually simpler: win overlay stays until New Game, but keys still work because overlay has pointer-events:none or I don't block. Let me set win overlay pointer-events none so clicks pass and keys work. For gameover, overlay blocks (canMove false anyway) and button.

Let me make overlay: a div covering board. For win, class 'win' (semi-transparent, pointer-events none). For gameover, class 'gameover' (solid, pointer-events auto) with button.

I'll implement showWin: overlayEl.textContent='2 0 4 8!'; overlayEl.className='overlay win'; overlayEl.style.pointerEvents='none'. hide on next move: overlayEl.classList.add('hidden') or remove.

showGameOver: overlayEl.innerHTML = '<div>Game Over</div><button class="button" id="ovNewGame">New Game</button>'; overlayEl.className='overlay gameover'; addEventListener to button.

Let me keep it simpler: overlay has a title and optional button. I'll set content and a button that calls resetGame.

For hiding win overlay on next move: in handleMove after result.moved true, if won and overlay visible win → hide. Or check: if overlay.classList contains 'win' → remove. Let me, in handleMove, before animating, if overlay showing win → hide overlay (allow continue). Actually let me hide win overlay when a move is made: in handleMove, `if (won && !overlayHidden) hideWinOverlay()`. Let me just call hideOverlayIfWin() at start of handleMove when moved. I'll implement.

This is a lot. Let me just write the code and test.

Let me also add: the score should increase by merged value. In moveBoard, score accumulates merged tile value (doubled). For [2,2,2,2]→[4,4]: merges 4 and 4 → score +=8. ✓. For [2,2,4]→[4,4]: merge 4 → score +=4. ✓. Good, matches "score increases by the value of each new merged tile" (merged tile value).

Now let me write the file.

Let me also handle best score. I'll include localStorage 'best2048'. On score update, if score>best update. resetGame resets score but keeps best. Display best below score.

Let me write it all.

Let me also make sure to call preventDefault on arrow keys. Add window keydown listener.

Also prevent page scroll: HTML { overflow:hidden } and body no scroll. Arrow keys on document—preventDefault.

Let me write.

For tile text font size per value, classes handle it. For values beyond 8192 (theoretically up to huge if continue), default class dark. I'll make tileClass return 'vX' for known, else 'v8192' (dark) — but value text might be huge; font-size 28. Acceptable.

Let me write tileClass:
```js
const COLORS = {2:'v2',4:'v4',8:'v8',16:'v16',32:'v32',64:'v64',128:'v128',256:'v256',512:'v512',1024:'v1024',2048:'v2048',4096:'v4096',8192:'v8192'};
function tileClass(v){ return COLORS[v] || 'v8192'; }
```

Now let me write everything in the file.

Let me also add a touch of: when a tile moves, slight delay so all move together. Fine.

Let me also handle the case where result.moved but merges empty and some tile slides—no mergedIds, fine, new tiles = spawned only.

Now, one concern: `animateMove` fadeOutAndRemove uses a CSS transition on opacity then removes. I'll add class 'fadeout' and setTimeout to remove element after. But elements being repositioned (slide) during the same frame—fine.

Also, the moved tiles slide via left/top transition. The merge sources fade out. The merged/spawn tiles appear via animation. The durations: slide .16s, appear .18s, mergePop .22s, fadeout .18s. Lockout 280ms.

Let me write fadeOutAndRemove:
```js
function fadeOutAndRemove(el){
  el.classList.add('fadeout');
  setTimeout(()=> { if (el.parentNode) el.parentNode.removeChild(el); }, 200);
}
```
CSS .fadeout { transition: opacity .18s ease; opacity:0; } But I also need to not interfere with position. fine.

Wait, if a tile element is being faded out AND it had 'new' class with transform animation, adding fadeout changes opacity transition. Minor. Fine.

Let me finalize. Also ensure boardEl is position:relative.

Let me write the complete file now.

Actually, let me reconsider the appear animation using transform while the tile also has left/top transition. For new tiles I don't slide them (placed at final pos), so only 'new'/scale animation. Good. For merge-pop, scale animation at final position. Good.

But there's a catch: when a tile is created with class 'new' and I set left/top, no transition issue (new element, transition from initial? Actually setting left/top on a newly added element—since it's new, there's no "from", and transition wouldn't fire for the initial set because there's no prior value to transition from in a visible way... actually transitions don't fire on initial render properties if the element was just added; the appear animation is separate via transform). Good.

Now let me also ensure that for slid tiles, the transition works: element already in DOM with left/top set previously; I change left/top → transition animates. Good. I must NOT have 'new'/'merge-pop' on a slid tile (I reset className to 'tile vX'). Good.

Alright, writing now.

Let me also double-check: initial board render. After resetGame (or init), I create board array, spawn 2 tiles, render initial. Initial tiles: create el with 'new' class (appear) for nice effect. Let me do that.

Let me write initGame:
```js
function initGame(){
  board = Array.from({length:SIZE}, () => Array(SIZE).fill(null));
  tiles = new Map();
  score = 0; won = false; gameOver = false;
  // clear boardEl children except grid and overlay
  while (boardEl.firstChild) { if (boardEl.firstChild.classList && (boardEl.firstChild.classList.contains('grid')||boardEl.firstChild.classList.contains('overlay'))) { boardEl.appendChild(boardEl.removeChild(boardEl.firstChild)); } ... }
}
```
Simpler: keep grid and overlay as separate elements outside or with ids; clear tiles by removing elements with class 'tile'. Let me select all .tile and remove. And clear tiles map.

Let me structure HTML: boardEl contains gridEl and overlayEl; tiles appended as .tile children. To reset, remove all .tile children.

```js
function removeAllTiles(){
  document.querySelectorAll('.tile').forEach(el=>el.remove());
  tiles.clear();
}
```

initGame:
```js
function initGame(){
  board = Array.from({length:SIZE},()=>Array(SIZE).fill(null));
  tiles = new Map();
  score=0; won=false; gameOver=false;
  removeAllTiles();
  hideOverlay();
  scoreEl.textContent='0'; bestEl...
  spawnTile(); spawnTile(); // two tiles
  renderInitial();
}
```
renderInitial: for each tile object, create el with 'new' class, position.

spawnTile: pick empty cell, create {value,id}, board set, return tile; create el in render.

Let me make spawnTile not create el; just set board. Then render creates el. For initial, I'll render all tiles. For move, animateMove creates els for new tiles.

Let me unify: a function renderBoard() that clears and recreates all tile els—NO, that loses animation. I need incremental. For initial, creating all with 'new' is fine. Let me write renderInitial that iterates board and creates el per tile with 'new' class.

For animateMove, as designed.

Let me ensure tile ids persist across moves: board tile objects carry id; tiles map keyed by id. On move, moveBoard produces newBoard sharing non-merged tile objects (same ids) → tiles map entries persist by id. I reposition. Good.

But initial tile objects: spawnTile creates {value,id} and sets board[r][c]. tiles map populated in renderInitial with el and id. On first move, moveBoard reuses these objects' ids → tiles.has(id) true → reposition. Good.

Let me write renderInitial:
```js
function renderInitial(){
  for (let r=0;r<SIZE;r++) for (let c=0;c<SIZE;c++){
    const cell=board[r][c];
    if (!cell) continue;
    const el=document.createElement('div');
    el.className='tile '+tileClass(cell.value)+' new';
    el.textContent=cell.value;
    el.dataset.id=cell.id;
    const off=cellOffset(r,c);
    el.style.left=off.x+'px'; el.style.top=off.y+'px';
    boardEl.appendChild(el);
    tiles.set(cell.id,{value:cell.value,r,c,el});
  }
}
```

But I want gridEl and overlayEl not removed. removeAllTiles removes only .tile. Good. But removeAllTiles then renderInitial: gridEl and overlayEl remain as children; I append .tile after. Order: grid (background) then tiles then overlay. overlay z-index high. Let me ensure overlay is last (highest z). I'll append overlay after tiles or set z-index. Let me set overlay z-index 10. And tiles z-index 1, grid z-index 0 (or default). Actually grid is a flex/grid child; tiles absolute positioned in boardEl (relative). Need grid to be in flow and tiles absolute over it. boardEl position relative; gridEl position? If gridEl is in normal flow, tiles absolute positioned relative to boardEl (the positioned ancestor) overlap grid. Good. overlayEl absolute covering boardEl. Let me set overlayEl position absolute inset 0.

But if gridEl and tiles both children of boardEl and tiles are position:absolute, they overlay gridEl which is in flow. Good. Order in DOM: gridEl first (background), then tiles, then overlayEl. I'll ensure overlayEl last.

Let me structure HTML with boardEl containing grid container and overlay. Tiles appended into boardEl. To keep overlay on top always, z-index. I'll set overlay z-index 10, tiles z-index per value (or just 1). Let me set tile z-index = value rank to avoid overlap issues; not critical. I'll set all tiles z-index 2, overlay 10.

Hmm but slid tiles overlap during move; higher z-index for active. Not critical for correctness. I'll set tile z-index = Math.log2(value) so higher tiles on top—nice touch. Set in JS: el.style.zIndex = String(Math.log2(cell.value)). For merged/pop, maybe higher. Let me just set zIndex = value's log2; merged tile gets log2 too. Fine.

Let me now also handle the score update display and best.

Time to write. Let me also add a "score scored flash" quickly: when score increases, add class to scoreBox animating color. I'll add CSS animation. Optional. I'll add a quick one.

Let me write the file.

I realize I should double check the `moved` detection in slideLineObjs once more for a pure slide (no merge): row [0,2,0,0] left: cells=[{0},{2},{0},{0}]. compacted=[{2@(0,1)}]. out: push tile 2 at front cells[0]=(r,0). i=1 end. pad nulls. out=[{tile2,r0c0},{null},{null},{null}]. cells[0].tile was null. movedCheck: cells[0] null vs out tile2 → moved true. Good. Result board row [2,0,0,0]. ✓.

Another: [2,0,0,0] left: cells=[{2},{0},{0},{0}]. compacted=[{2}]. out=[{tile2@(r,0)},null,null,null]. movedCheck cells[0].tile=2 vs out tile2 same id? out tile = compacted[0].tile = same object id. cells[0].tile is the SAME object (board[0][0]). So id same → not moved at idx0. idx1: cells[1] null vs out null → false. So movedCheck false. Good (stable). 

Now [2,2,0,0] left: cells=[{2a},{2b},{0},{0}]. compacted=[{2a},{2b}]. i=0: 2a.value==2b.value → merge → merged 4 (new id) at cells[0]=(r,0), sources (r,0),(r,1). i=2. out=[{merged4@(r,0)},null,null,null]. movedCheck: cells[0].tile=2a (id a) vs out tile merged id new → moved true. Good. Result [4,0,0,0]. ✓

Now [2,4,0,0] left: cells=[{2},{4},{0},{0}]. compacted same. i=0:2≠4→out[0]=2@(r,0). i=1: out[1]=4@(r,1). Wait out positions: out[0] front cells[0]=(r,0); out[1] front cells[1]=(r,1). But 4 was originally at (r,1) already; it stays. movedCheck: cells[0]=2 vs out[0]=2 same id → false. cells[1]=4 vs out[1]=4 same → false. So moved false! But is that correct? [2,4,0,0] left → no change (already compact, no merge). Correct, moved false. ✓ Good, no spawn.

Now [4,2,0,0] left: compacted=[4,2]. i=0:4≠2→out[0]=4@(r,0), out[1]=2@(r,1). But originally 4 at (r,0), 2 at (r,1) → unchanged. moved false. Correct.

[0,4,2,0] left: cells=[0,4,2,0]. compacted=[4@(r,1),2@(r,2)]. out[0] front=(r,0) tile4, out[1] front=(r,1) tile2. movedCheck: cells[0] null vs out[0] tile4 → moved true. cells[1] tile4 (id) vs out[1] tile2 (id) → differ → moved. Result [4,2,0,0]. ✓ 

Now a tricky merge case verifying the "merge once per tile" rule: [2,2,2,0] left → should be [4,2,0,0]? Let me check 2048 rules: [2,2,2,0] left. The first two 2's merge into 4; the third 2 stays (can't merge with the just-merged 4). Result [4,2,0,0]. Score +4. 

My slideLineObjs: cells=[2a,2b,2c,0]. compacted=[2a,2b,2c]. i=0: 2a==2b → merge 4 at cells[0]=(r,0), sources (r,0),(r,1). i+=2 → i=2. i=2: 2c alone → out[1]=2c@(r,1). pad. out=[4,(r,0), 2c,(r,1), null,null]. Result [4,2,0,0]. ✓ merged once. 

And [2,2,2,2] → [4,4,0,0] (verified earlier). ✓.

Great, logic correct.

Now let me also verify moveBoard writes back correctly for 'right' direction. dir='right', line cells front-to-back = right to left: for (let c=SIZE-1;c>=0;c--) cells.push({tile:board[i][c], r:i, c}). So cells[0] = rightmost. After slideLineObjs, out[k] at front = cells[k] position. out[0] (front) = rightmost cell (c=3). So result: tiles packed to the right. Good. newBoard[out[k].r][out[k].c] = tile. For right, the merged tile lands at cells[0] = (i,3) — rightmost. Correct.

Example row [2,2,0,0] right → should be [0,0,0,4]? Wait [2,2,0,0] right: pack 2,2 to right → [0,0,2,2], then merge → [0,0,0,4]. Let me verify: cells front-to-back right-first = [board[i][3]=0, board[i][2]=0, board[i][1]=2, board[i][0]=2] = values [0,0,2,2]. compacted=[2@(i,1),2@(i,0)]. i=0: 2==2 merge → 4 at cells[0]=(i,3), sources (i,1),(i,0). i=2. out=[4@(i,3), null,null,null]. newBoard: (i,3)=4, others null→0. Result row [0,0,0,4]. ✓. Merge at rightmost. 

Now 'up' and 'down' analogous. Good.

Let me write the file.

I'll also add sound? No network, no audio. Skip.

Let me also add on-screen arrow buttons? Not required; keyboard verified. But browser tool might simulate keydown via the `key` action. I'll rely on keydown. Let me also add click handlers on cells? No.

Let me write now. Then run in browser via the browser tool, inspect, and do a short play-through.

Let me write the complete index.html.

For best score display, I'll add a best-box.

Let me also ensure the page doesn't zoom/scroll on arrow keys: body overflow hidden, html height 100%. preventDefault arrows.

Also the browser might need the keydown to reach window. I'll add listener on document with {passive:false}.

Now writing.

Let me write the tile move transition and a "scored" flash for score.

CSS for score flash:
```
.score-box.scored .value { animation: scorePulse .18s ease; }
@keyframes scorePulse { 0% { color:#f9f6f2; } 50% { color:#fff58f; } 100% { color:#f9f6f2; } }
```
Hmm color change. Fine. I'll add class 'scored' on scoreBox when scoring, remove after.

Let me finalize code. I'll write it.

Actually, the merged tile's value: in animateMove, for a merged tile I create el with merge-pop and textContent = cell.value (the doubled value). Good. The two source elements fade out (their ids removed). Good.

But careful: a merged tile's id is new; in the else branch (not in tiles map) → I check mergedIds.has(cell.id) → 'merge-pop'. But I also might have set spawned and check spawned.id===cell.id for 'new'. Order: if merged → merge-pop; else if spawned → new; else (shouldn't happen for new tile). Let me code: 
```
let anim = '';
if (mergedIds.has(cell.id)) anim='merge-pop';
else if (spawned && spawned.id===cell.id) anim='new';
el.className='tile '+tileClass(cell.value)+' '+anim;
```
But spawned tile is never a merged tile (spawn happens after move; merged tiles are from result.merges). And merged ids are in newIds (board has them). spawned.id also in newIds. They're disjoint sets. Good.

Wait, is the spawned tile potentially overlapping a merge cell? Merge cells are filled by merged tiles; spawn goes to a random EMPTY cell. After move, empty cells are those not occupied. The merged tile occupies its target cell (was source cells before, now target). So spawn targets a different empty cell. Could spawn land on... no, spawn picks empty. Good, no overlap.

Edge: board full after move (no empty) → no spawn. Then only merges/slides animate. Good. Check game over after.

Alright, writing the file now for real.

Let me also make sure to set canMove appropriately during win overlay: after win, keys still work (canMove true). Game over: canMove false (set after animation, and isGameOver → gameOver true, handleMove returns early if gameOver). Good.

Let me write.

Also: the overlay for win — I'll keep it visible but allow moves; on next move that changes board, hide win overlay. Let me in handleMove: if won && result.moved and overlay is win-visible → hideOverlay(). Actually let me hide win overlay right when a new move starts (so user can see board continue). I'll call maybeHideWinOverlay() at start of a successful move.

Let me implement hideOverlay() sets overlayEl.classList.add('hidden') and clears content. showWin sets content. 

For gameover overlay, I'll keep it until New Game (canMove false prevents moves). Include New Game button.

Let me write final code.

I think I've planned enough. Let me write the file.

Let me write it with clear structure. I'll include comments.

After writing, I'll run in browser tool, inspect, then play: simulate arrow keys (I'll use the browser tool's key action or just test via JS console calls to handleMove, or actually the browser tool supports key action? The tools include `key` action for browser. Let me use browser tool to press arrows? The browser tool `key` action types text; for arrow keys I can send key 'ArrowLeft' etc. Let me use that. Actually the `key` action description: "type" — likely types characters. For arrow keys, maybe key='ArrowLeft'. I'll try. Alternatively, I can test logic via console by calling handleMove('left') etc. But better to test real keydown path. Let me use browser key action with ArrowLeft etc. If that doesn't work, I'll dispatch keydown via console.

Let me write the file, then open in browser, inspect console for errors, then play a sequence.

Let me write now.

Let me reconsider cellOffset and ensure tiles appear within board. I'll trust the math.

Let me also set boardEl position relative and size 448x448. gridEl fills. overlayEl absolute inset:0.

Writing:

I made a mistake in animateMove: I called positionTile with wrong r,c twice. Let me fix that. The first positionTile call with `r: c` is wrong. Let me fix the file before running.

Also in `onKey`, the gameOver branch has a syntax error (`...`). And I used `...` in code which is invalid. Let me fix.

Let me fix animateMove:

```js
if (tiles.has(cell.id)) {
  positionTile({ value: cell.value, id: cell.id, r, c });
}
```
remove the bad first call.

And onKey: the gameOver branch — just `return;` already handles it (returns before handleMove). Let me clean up onKey to remove the broken part.

Let me edit the file.

I used the wrong parameter format. The edit function uses `edits` as an array of {oldText, newText}. Let me redo.

Now fix the onKey gameOver branch syntax error.

The page loaded with two 2 tiles, score 0, best 0. No JS errors in console (only a sandbox warning). Let me check the console more carefully and the DOM. Let me inspect.

Let me look at the console for errors and the grid layout. The DOM shows "2 2" so two tiles spawned. Let me verify the board rendering and then play a sequence.

Let me check console errors specifically.

Let me use the browser inspect action to see console errors and the rendered tiles.

No errors. The board shows two 2 tiles. Let me check the positions of tiles to verify layout is right. Let me query the DOM for tile elements and their positions via console. I'll use the key action to simulate arrow presses. But the browser `key` action might not send ArrowLeft properly. Let me test by pressing ArrowLeft key.

Actually, let me first verify the board state via a quick check in the console. But I don't have direct console access beyond what inspect returns. Let me try the key action to simulate arrow keys.

Let me try pressing "ArrowLeft" to slide left (should spawn a tile if board changes). Actually with two 2 tiles, sliding left won't merge unless they're in the same row. Let me play a few moves.

Let me try the key action with ArrowLeft, ArrowRight, etc. But the two tiles are randomly placed. Let me just press a sequence and observe the score/tile text changes via inspect.

Let me press ArrowDown a few times, then check.

Actually, the browser `key` action - I'm not 100% sure how it sends arrow keys. Let me try it. The description says "type" — for special keys it might accept the key name. Let me try "ArrowLeft".

The arrow key works. After pressing ArrowLeft once: score went to 4, BEST 4, and we now have tiles 2,2,2,4. Wait, that means a merge happened (2+2=4, score +4) and a new tile spawned (the 2). Wait let me think: initially two tiles (random positions). Pressing ArrowLeft once: if the two 2's were in the same row, they'd merge to 4 (score 4), then spawn a new tile. We see tiles 2,2,2,4 — that's four tiles. Originally two 2's merged into 4 (one tile), score 4, then spawn one 2 → total tiles: 4 and 2 = two tiles. But we see 2,2,2,4 = four tiles. 

Hmm, that doesn't add up. Let me reconsider. Maybe the initial two tiles weren't in the same row, so no merge. Then sliding left moves them (score stays 0), then spawn → three tiles. But we see score 4 and four tiles.

Wait, maybe the two initial tiles were both 2's in the same row plus... no, only two tiles initially.

Actually wait — maybe my `animateMove` is being called but also the spawn. Let me recount: Initially 2 tiles. Press ArrowLeft:
- If they merge: board had 2 tiles → merge to 1 tile (4) + spawn 1 tile = 2 tiles. Score 4. But DOM shows 4 tile values: 2,2,2,4. That's 4 tiles.

Hmm. Unless the initial board had more than 2 tiles, or spawn is happening multiple times.

Wait, let me re-read the DOM output. The `dom` field shows the text content of the page: "2048 SCORE 0 BEST 0 New Game 2 2 ...". The "2 2" were the two tiles. After one ArrowLeft: "2 2 2 4" — that's the footer text? No. The footer is "Use arrow keys...". 

Wait, actually the tile elements are divs with textContent = value. The DOM text dump includes all text: title, score, best, button, tile values, footer. Initially: title "2048", label "SCORE", value "0", label "BEST", value "0", button "New Game", then tile "2", tile "2", footer text. So "2 2" = two tiles. ✓.

After ArrowLeft: tiles show "2 2 2 4"? That's 4 tiles with values 2,2,2,4 and score 4. 

Hmm, how did we get 4 tiles from 2? If initial two 2's didn't merge (different rows), sliding left moves them, no merge, score 0, spawn one → 3 tiles. But score is 4 and 4 tiles. 

If initial two 2's merged: 2 tiles → 1 tile (4) + spawn 1 (2) = 2 tiles, score 4. But we see 4 tiles.

So something is off — possibly tiles aren't being removed properly (stale elements lingering), OR spawn is happening more than once, OR the merge left extra elements.

Wait — could it be that the initial render created tiles that aren't being cleaned? Or maybe `animateMove` creates new elements for moved tiles but doesn't remove old ones (because old id tiles are repositioned, not duplicated). Hmm.

Actually, let me reconsider: maybe both initial tiles were 2's AND in positions such that after left slide they merge, AND there was already... no.

Let me reconsider: maybe the issue is that `animateMove` is called but `positionTile` creates the element reposition, and for NEW tiles (merged/spawned) renderTile appends. The old elements for moved tiles are repositioned (same element). So tile count should match board count.

Unless... the initial two tiles: let me reconsider what "2 2" then "2 2 2 4". 

Actually maybe the first ArrowLeft didn't merge (score should be 0 then). But score is 4. So a merge happened.

Hold on — maybe my initial spawn created two tiles, but they were placed and one merge... Let me think about whether the board could have started with the two 2's adjacent such that left merge. With score 4 that means one 2-2 merge. After merge: one 4 + spawn one new (2 or 4). So 2 tiles total. But DOM shows 4 tiles (2,2,2,4). 

The only way to get 4 tiles: 2 initial, no removal, + spawn, + ... no. Or stale elements not removed.

Wait, maybe there are DUPLICATE elements because `collectIds()` / fade logic failed to remove merged sources, leaving stale elements. Let me reconsider: if the two 2's merged, source ids (the two original 2 tiles) should be removed via fadeOut (they're not in newIds). The merged 4 is new id → renderTile. The spawned tile (new id) → renderTile. Plus... wait, are there other tiles? No. So 2 elements. But DOM shows 4.

Hmm, unless the merge didn't actually consume both sources. Let me reconsider `moved` detection / `animateMove`.

Actually, wait. Let me reconsider: maybe the two initial tiles were NOT in the same row/col, so ArrowLeft moves them (sliding) with NO merge, score 0, spawn one → 3 tiles. But we got score 4 and 4 tiles. So that's not it either.

Let me reconsider: Could pressing ArrowLeft have triggered multiple moves or the spawn logic added multiple? Or could `animateMove`'s "remove disappeared tiles" loop be failing because newIds includes stale?

Actually, here's a possible bug: in `animateMove`, I compute `newIds = collectIds()` from the CURRENT board (after move + spawn). Then for tiles in the map not in newIds, fade out. For tiles in newIds that are already in map, reposition. For tiles in newIds not in map, renderTile.

But the merged sources: their ids are NOT in newIds (consumed). So they get faded out. Good. So no duplicates.

Wait, unless `moved` was true but actually no merge, yet... Let me reconsider. Score 4 means one merge of two 2's. After merge board has: merged 4 (new id), and the spawn. That's 2 tiles. Unless the original two 2's: one of them also slid to a new cell (different id)? No.

Hmm, let me reconsider whether maybe both initial tiles were 4? No, DOM showed "2 2".

Let me reconsider: maybe there were MORE than 2 tiles initially due to a bug in resetGame. resetGame spawns twice. But maybe spawnTile or renderTile double-counts? 

Actually, wait. Let me recount the final DOM: "2 2 2 4" — is it possible these are: two 2's (original, one didn't merge and stayed), plus merged 4, plus spawned 2? That's 4 tiles: original 2 (non-merged), merged 4, spawned 2, and... that's the one original 2 that didn't merge. So scenario: initial board had THREE 2's? No, resetGame spawns exactly two.

Unless... the very first ArrowLeft: board had two 2's. If they were in the same row adjacent, merge → 4 (1 tile) + spawn → 2 tiles. Not 4.

I'm confused. Let me actually look at the real state. Let me check the board array and tiles via a console log. I don't have a direct console, but maybe I can add a debug. Actually, let me reconsider the DOM text interpretation. 

"Use arrow keys to slide tiles. Reach 2048 to win. Keep playing after a win!" is the footer. The tile values: after move, "2", "2", "2", "4". 

Actually, maybe the initial two tiles were both 2 in the SAME column but different rows, so ArrowLeft doesn't merge (they're in different rows). Sliding left just repositions (no merge), score stays 0. But score is 4!

Unless sliding left caused a merge. For a merge on left, two equal tiles must be in the same ROW and adjacent (after compaction) in that row. With only 2 tiles, if both in same row, left slide merges them. So initial two 2's in same row → merge → score 4, 1 tile + spawn = 2 tiles.

So why 4 tiles? Let me hypothesize: STALE ELEMENT BUG. The two original 2's: one is the merge source (consumed, faded), the other is also a merge source (consumed, faded). The merged 4 created. Spawn created. So 2 elements. 

Unless the fade-out isn't removing and renderTile is creating NEW elements for tiles whose ids ARE in the map (repurpose)... no.

Wait, maybe the bug: `positionTile` is called for tiles in newIds that ARE in the map. But for the merged tile and spawn, they are NOT in the map (new ids), so renderTile creates them. Fine.

But what about the original two 2's? Both consumed → faded. So gone. 

I really can't reconcile 4 tiles. Let me actually inspect the DOM elements and board via console. Let me write a tiny debug by running JS. The browser tool doesn't give a JS console directly, but `inspect` returns the rendered DOM text. 

Alternatively, let me reconsider: maybe the initial render showed "2 2" but actually there were already 2 tiles, and pressing left when they're in the same row merges and spawns → the DOM now has the merged 4 and a new 2. That's 2 tiles: "4 2". But DOM shows "2 2 2 4". 

Hmm, unless the merge produced a 4 and BOTH original 2's stayed (didn't get removed) AND a spawn happened → 2 2 4 + 2(spawn) = "2 2 2 4". That's exactly it! The two original 2's did NOT get removed (stale), the merged 4 was created, and a spawn 2 was created. So stale elements = the two original 2's not removed.

Why would they not be removed? In animateMove, the fade-out loop: `for (const [id,t] of tiles) if (!newIds.has(id)) fadeOut`. The original 2's have ids that, after a merge, are NOT in newIds. So they SHOULD be faded. Unless newIds still contains them.

OH. I see a bug. In `handleMove`:
```js
board = result.board;
score += result.score;
const spawned = spawnTile();
animateMove(result, spawned);
```
spawnTile adds a new tile to board. Then animateMove computes newIds from board (which includes spawned). The merged sources are gone from board. So newIds = {merged4 id, spawn id}. The original two 2 ids are NOT in newIds → faded. Good.

Unless result.merged detection failed and the two 2's did NOT actually merge, but score incremented anyway? No, score only increments in slideLine on merge.

Hmm, wait. Let me reconsider: maybe the two original 2's did NOT merge (different rows), so ArrowLeft just slides (no merge, score 0)... but score is 4. Contradiction.

Unless MORE moves happened. Did pressing once cause multiple? No, one keydown = one handleMove.

Let me just actually inspect the live board. Let me add temporary console.logs and reload, then check console via inspect. Actually inspect showed only the sandbox warning, no logs (because I didn't add logs). Let me add a debug log in handleMove to print board values and tile count, then play, then inspect console.

Actually, let me reconsider the DOM dump format. Maybe the "2 2 2 4" includes tile values but ALSO the score "4" is appended as text? No, scoreEl shows "4" already in "SCORE 4". The order in DOM: header (title, SCORE label, score 4, BEST label, best 4, New Game button), then board (tiles + overlay), then footer. So after "New Game" we have tile texts then footer "Use arrow keys...". So "2 2 2 4 Use arrow keys..." = four tiles 2,2,2,4.

Wait, but BEST is 4 now (was 0). Score 4. Four tiles 2,2,2,4.

So board has tiles valued 2,2,2,4 = sum 10. Started with two 2's (sum 4). After one move: if merge 2+2=4 (sum 4) + spawn 2 (sum 2) = sum 6, tiles 4,2. But we have sum 10 (2+2+2+4). That's two 2's (sum4) + merged 4 (sum4) + spawn 2 (sum2) = sum10, 4 tiles. 

So it's: original two 2's BOTH remained (stale, not removed), merged 4 created, spawn 2 created. So the merge logic created a NEW merged tile but did NOT remove/consume the source tiles from the board.

That means in moveBoard/slideLine, the merge created the merged tile at the target, but the SOURCE tiles were NOT cleared from their cells. Let me check slideLine and moveBoard.

In slideLine:
- compacted = cells.filter non-null. Sources removed from consideration but...
- out is built: for merge, out.push({tile: merged, r, c}) and i+=2 (skipping the second source). For non-merge, out.push({tile: compacted[i].tile}).
- out padded.
- front-aligned positions assigned.
- Then return {out,...}.

In moveBoard: `next[res.out[k].r][res.out[k].c] = res.out[k].tile;` for all k. This writes the out tiles to their front positions. But it does NOT clear the cells that were occupied by sources in their original positions! Because `next` is a copy of b, and b had tiles at source positions. Writing out tiles to front positions overwrites front positions but the source positions (other cells) still hold their old tile objects from the copy!

That's the bug. For example, row [2a,2b,0,0] left: cells front-to-back left = [2a@0,2b@1,0,0]. compacted=[2a,2b]. merge → out=[{merged4 @cells[0]=(r,0)}, null@cells[1], null@cells[2], null@cells[3]]. Then next[r][0]=merged4, next[r][1]=null, next[r][2]=null, next[r][3]=null. Wait, the loop `for k: next[out[k].r][out[k].c] = out[k].tile`. out[1].tile=null → next[r][1]=null. out[2].tile=null → next[r][2]=null. out[3] null → next[r][3]=null. So sources at original (r,0) and (r,1) are overwritten: next[r][0]=merged4 (overwrote 2a), next[r][1]=null (overwrote 2b). 

But this only works if the line is a ROW. For 'left', cells = row cells in order, out front-aligned maps to columns 0,1,2,3. Writing next[r][out[k].c] covers all 4 columns. So row fully rewritten. ✓.

For 'right': cells front-to-back = columns 3,2,1,0. out front-aligned: out[0]→cells[0]=(r,3), out[1]→cells[1]=(r,2), etc. Writing next[r][out[k].c] covers columns 3,2,1,0 = all columns. ✓.

For 'up': cells = column i rows 0,1,2,3 (front= top, r0). out[k]→(r=k, c=i). Writing next[out[k].r][out[k].c] covers rows 0..3 of column i. ✓.

For 'down': cells front-to-back = rows 3,2,1,0. out[k]→cells[k]. Writing covers all rows of column i. ✓.

So EVERY line rewrites ALL cells of that line (row or column) with out values (including nulls). So source positions should be cleared. 

So why stale? Let me re-examine. Oh wait—maybe the issue is that the two original 2's were in DIFFERENT rows, so ArrowLeft only affected their respective rows, each row had one 2 → slides to col 0 (no merge, score 0). But score is 4! So a merge DID happen.

Hmm, so a merge happened (score 4). For a merge, two equal tiles in same row. With two tiles total, both 2's in same row → merge. That rewrites that row fully. The OTHER row untouched. So board after: merged 4 at col0 of that row, other cells 0, and the OTHER original 2 (in the other row or same row col1?)...

Wait, if both 2's in same row, there are only 2 tiles, both in that row. Merge → row becomes [4] (1 tile). Then spawn adds 1 tile elsewhere → 2 tiles. So 2 tiles total. Not 4.

Unless the two 2's were in the SAME row but the merge only consumed... no.

I'm clearly missing something. Let me just inspect the actual board array and element count with debug logs. Let me add console.log in handleMove and animateMove temporarily, reload, press key, inspect.

Actually, faster: let me reconsider — maybe pressing ArrowLeft the FIRST time, the board had 2 tiles NOT in same row (no merge), slide (score 0), spawn → 3 tiles. But DOM shows score 4. So that's not it.

OR maybe the FIRST keydown triggered merge. Let me just debug. Let me add a log.

Actually, wait. Let me reconsider the spawn. `spawnTile()` is called in handleMove AFTER `board = result.board`. But result.board for a non-merged move (just slide) — fine. For a merged move, result.board has merged tile. spawnTile picks an empty cell. After a 2-into-4 merge starting from 2 tiles in same row: board has 1 tile (the 4) at (r,0), all else empty. spawnTile picks a random empty cell → adds a 2. Now board has 2 tiles: 4 and 2. animateMove creates/updates elements: newIds = {4id, 2(spawn)id}. The original two 2 ids NOT in newIds → faded. merged 4 (new id) rendered. spawn 2 rendered. So 2 elements. 

Unless fadeOut removed them from the map but the elements... fadeOut just adds class and removes after 220ms; the el is removed from DOM. So after animation, 2 elements. But DOM snapshot was taken right after keydown (before 220ms timeout). At that instant, fadeOut added 'fadeout' class but element still in DOM (opacity still 1 until next frame, and remove at 220ms). So at snapshot time, the two original 2 elements are STILL in DOM (not yet removed) AND the merged 4 and spawn 2 are also rendered. So 4 elements temporarily! 

That explains it. The initial DOM snapshot was captured immediately after the key event, before the 220ms fade-out removal completed. So at that moment: 2 stale (fading) + 4 + 2(spawn) = 4 elements visible (the stale ones still fully visible since opacity transition hadn't progressed). 

So this is likely NOT a bug — just timing of the snapshot. Let me wait and re-inspect to confirm stale elements disappear.

Let me press a few more keys with delays and check final state. Let me also verify merge/score via a clearer test: I'll try to force a known scenario.

Given 5 min left, let me do a targeted verification: 
1. Confirm stale elements removed after timeout (re-inspect after delay).
2. Verify a merge updates score by merged value and tiles reduce.

Let me press ArrowLeft a few times and then inspect after a pause. Actually I can't pause easily. Let me press a sequence allowing animations between. Let me press ArrowDown, then inspect. Then maybe repeat.

Actually, let me reconsider the score flash / the "scored" flash. Also there's the `checkGameState` showing a hint overlay that auto-hides — that might obscure. The overlay hint shows "Press arrow keys" for 900ms. That's fine but the `showOverlay('hint',...autoHide 900)` adds an overlay which has pointer-events? The hint overlay class — I set overlayEl.className='overlay hint'; no pointer-events:none, so it might block clicks but keydown still works (keys are on window). Fine. But the hint overlay covers the board visually. It auto-hides after 900ms. OK.

Let me verify by playing more and inspecting. Let me press ArrowRight then inspect. Let me also double check the win detection threshold (>=2048) and game over.

Given time, let me press a couple more arrows and inspect to see if tile count settles to correct and score increments properly. Let me press Down.

Now DOM shows tiles "2 4 2" — three tiles: 2, 4, 2. Score 4. So after ArrowDown (from the previous state), the stale elements faded out and we have 3 tiles. The 4 (merged) stayed, the spawn 2 moved down (ArrowDown slid the 2's down), and a new 2 spawned. Score still 4 (no new merge in this move). 

Wait, previous state had 4 elements (2 stale + 4 merged + 2 spawn). After ArrowDown: the stale 2's were faded/removed. The remaining real tiles: 4 and 2 (spawn). ArrowDown slid them down (both in their column? depends positions). Then a merge? No new merge (score still 4). Spawn added a 2 → now 4,2,2? But DOM shows 2,4,2 = three tiles: values 2,4,2.

Hmm, let me think. After first move: board had merged 4 + spawn 2 (2 tiles, but DOM showed 4 due to fade). After ArrowDown: slide down. If the 4 and 2 were in different columns, they each slide down (no merge), score 4, spawn → 3 tiles: 4,2,2. But DOM shows 2,4,2 (three tiles: 2,4,2). That's 4,2,2 just reordered in DOM text (DOM order = append order, not grid position). So three tiles: one 4 and two 2's. Sum = 8. Started sum 4 (two 2's). Merge 2+2=4 (score 4, sum 4). Then spawn 2 (sum 6). Then ArrowDown slide (sum 6). Spawn 2 (sum 8). Three tiles: 4,2,2. ✓✓✓

So score 4 = one merge total (the first). Correct! And tile count correct (stale removed). The mechanics work.

Wait, but after ArrowDown with two 2's present and the 4, if two 2's were in the same column, ArrowDown would merge them → score +4 = 8. But score stayed 4. So no merge in ArrowDown. That's plausible if the two 2's weren't in the same column/row appropriately. Fine.

Great, so the game is working. The earlier 4-tile display was just fade-out timing. 

Now let me verify win and game over and new game within remaining time. Win requires reaching 2048 which takes many moves — I can't realistically play to 2048 by hand in 5 min. But I can verify the win detection works by a quick check. Let me instead verify via a forced scenario? I can't easily inject tiles without editing. 

Let me at least verify New Game resets: press New Game (click) and check board returns to 2 tiles, score 0.

Let me also verify arrow keys don't scroll and the game responds. 

Let me click New Game. The browser tool has a `click` action with x,y. The New Game button is in the header. Let me find its coordinates. The header is centered. Button "New Game" — roughly in the middle right area. Hard to hit exactly. Alternatively, I can dispatch a click via... I don't have a JS console. 

Hmm. Let me instead test New Game via keyboard? New Game isn't keyboard-bound. But resetGame is also bound to overlay button. 

Actually, I could test New Game by reloading (which calls resetGame on load) — but that doesn't test the button click. 

Let me try clicking at an estimated position for the New Game button. The button is in .score-panel after best box. Layout: title "2048" on left, then score, best, New Game. Centered header. Hard to estimate.

Alternatively, I realize I should make New Game also triggerable to verify. But the verification requirement: "Use New Game and check that the board returns to two tiles with a reset score." I need to actually click it.

Let me estimate the button position. The page is 1200x800 viewport (browser tool says 1200x800). The wrapper is centered, padding-top 26px. Header is flex centered. Board is 448x448 centered below header.

Widths: title (~maybe 180px), score-box, best-box, button. Total header width ≈ let me estimate: title "2048" ~160px, score-box ~110px, best-box ~110px, button ~130px. Plus gaps. Total ~160+110+110+130+36(gaps) ≈ 546px. Centered at 600 (center of 1200) → header spans ~327..873. Button is rightmost ≈ at x 743..873, button click target center ~790, y ~ top 26 + header height ~62 → y~60.

Button click area roughly x:[740,870] y:[25,75]. Let me click around x=800,y=55. Actually risky. 

Alternatively, I can verify New Game reset logic by examining code correctness, and test via a keyboard approach. But the task explicitly wants me to use New Game.

Let me try clicking at a reasonable spot and then inspect. Let me click x=800, y=55. If that misses, I'll adjust.

Actually, let me reconsider — maybe I can verify New Game differently. The resetGame is called on load (initial state is 2 tiles, score 0). And it's also called for the overlay New Game button. To test the BUTTON path, clicking is needed.

Let me attempt the click. Let me first screenshot to gauge positions? The browser returns images as "model does not support images" — so I can't see screenshots. I only get DOM text and console.

Hmm. So I can't visually locate the button. I'll estimate. Let me click around the likely button area. Given 1200px width, centered content. Let me compute more precisely.

Title "2048": font-size 62px, font-weight 280. "2048" width at 62px ≈ 62 * ~2.0 (digits wide) ... "2048" ~ 5 chars? '2','0','4','8' = 4 digits. At 62px, digit width ~ 0.6*62=37, 4 digits ~148px, plus letter-spacing 2px*3=6 → ~154px. So title ~154px wide, centered.

score-panel: score-box min-width 96, best-box min-width 96, button ~130. Plus label/value. Let me estimate score-box width: "SCORE" label (12px) + "0" value (30px) + padding 22*2=44 → content ~ 70 + 44 = 114, min 96 → ~114. best similar ~114. button: "New Game" at 20px ~ 100 + padding 26*2=52 → ~152? Let me say ~150. gaps: header gap 18px between title/scorepanel. score-box gap 14, score-panel gap 14.

Total header width ≈ 154 (title) + 18 (gap) + 114 (score) + 14 + 114 (best) + 14 + 150 (button) = 154+18+114+14+114+14+150 = 580. Centered at 600 → left ~310, right ~890. Title at 310..464. score-box at 482..596. best at 610..724. button at 738..888. Button center ~813, y ~ 26(pad-top)+ (header height/2). Header height: title 62 + line-height; score boxes: label 12 + value 30 + padding 10*2 = 72, so ~72 tall. centered → y from 26 to 26+? wrapper padding-top 26, then header. Header height ≈ max(title height ~70, scorebox ~72) ≈ 72. So button y ≈ 26 + 36 = 62 (vertical center) → button spans y ~26..98. Click y=62, x=813.

This is very approximate. Let me click x=810, y=62. If wrong, the click might hit nothing or the board (board is below header, y ~ 26+72+16 = 114 onwards). Clicking at y=62 is in header region. x=810 should be in button region.

Let me try clicking there and inspect.

Actually, I worry about mis-clicking. Let me instead ALSO add keyboard support for New Game (e.g., Enter or 'N' or 'R') to make it testable, then test reset via keyboard. But the task says "Use New Game" implying the button. Adding a key won't hurt and lets me verify reset logic. But verifying the button specifically is better.

Let me just click the button region and check. Let me click x=810,y=60.

Actually, let me reconsider the coordinate system: "Coordinates use a 1200×800 viewport." Click x,y.

Let me click at the button. I'll try x=810, y=60.

Wait, actually, a cleaner test: I can verify New Game by pressing a key that triggers resetGame? I don't have one. But I could add 'n' or 'Enter' to call resetGame, making it keyboard-testable AND keep the button. The task requires the New Game button to exist and work; testing reset logic via keyboard is a reasonable proxy but not exactly "Use New Game."

Let me both: add Enter/'n' fallback AND click the button. Let me click first; if it works, great.

Let me click x=810, y=60.

Hmm, actually let me reconsider whether clicking the board region (y~114+) would do anything — board click does nothing (tiles are pointer-events:none, board no click handler). So a misclick on board is harmless.

Let me try clicking the button.

New Game clicked → board reset to two 2 tiles, score 0, best persists at 4. So resetGame works. 

Now let me verify:
1. Arrow keys don't scroll (page has overflow hidden + preventDefault). Can't easily verify scroll behavior, but HTML/body overflow hidden and preventDefault on arrows. Good enough.
2. Win on reaching 2048 — can't easily reach by playing. But the code checks `value >= 2048` and shows overlay, allowing continue. Let me trust it but maybe verify by a quick forced test.

Let me verify the win detection and continue-after-win with a quick code-based test. I could temporarily set a tile to 2048 via... I can't inject easily. 

Alternative: I can verify the logic by reasoning, but the task wants verification. Given 4 minutes left, let me do a quick forced win test by editing the code to, e.g., start with a 2048 tile? No—that breaks the real game. 

Better: Let me confirm win detection logic via a one-off: I could add a hidden debug key (e.g., press '0' to set a tile to 2048) but that's hacky and I'd need to remove it. 

Actually, let me verify win detection works by playing enough to get a high tile is impossible in time. Let me instead trust the logic and do a focused check: I'll verify that hasWon() triggers when a 2048 exists by temporarily testing in console. But I have no console access except via the DOM dump.

Hmm. Let me reconsider: I can run a quick JS snippet? The browser tool doesn't expose eval. But maybe I can add a temporary test in the HTML that auto-runs? That'd require editing and reloading.

Given time constraints, let me do a pragmatic verification of win/game-over via a temporary in-code test: I'll add a hidden keypress '9' that forces a win scenario (place 2048) — no, that changes game.

Alternative: I'm fairly confident in the win/gameover logic. Let me at least verify game over detection by reasoning and maybe a quick stress: I can't easily fill the board.

Let me instead make a focused, time-efficient verification:
- Merge score update: ✓ (confirmed score 4 from one 2+2 merge).
- Slide + spawn: ✓ (tiles moved and spawned).
- New Game reset: ✓ (score 0, 2 tiles).
- Win message: logic checks >=2048, shows overlay 'win', allows continue (won flag, keys still work, hide overlay on next move).
- Game Over: isGameOver checks no empty + no moves possible.

Let me quickly verify win by a temporary trick: I'll temporarily edit the spawn to occasionally be 2048? No.

Actually, let me just verify the win overlay code path by adding a temporary hidden key handler 'p' that sets a tile to 2048 and triggers checkGameState, test it, then remove it. This lets me verify the win overlay appears and continue works. Given ~4 min, let me do it efficiently.

Actually, simpler and safer: let me add a console.assert-style self-test? No.

Let me do the temporary 'p' key: add `if (e.key==='p') { forceWin(); }` temporary. forceWin sets a cell to 2048 and calls checkGameState. I'll verify overlay appears, then press an arrow to dismiss and continue, then remove the debug code.

Let me add this debug, reload, press p, inspect, then remove. Time-box: quick.

Actually, even simpler: I can verify by temporarily changing the win threshold? No.

Let me just add the debug key, test win + game over, and remove. Let me edit.

Actually, let me reconsider time. I have ~4 min. Let me add a debug helper that's easy to remove. I'll add right before the keydown listener:

```js
// DEBUG
function forceWin(){ for(let r=0;r<SIZE;r++)for(let c=0;c<SIZE;c++){ if(!board[r][c]){ board[r][c]={value:2048,id:nextId()}; break; } } }
```
And in onKey, handle 'p' to forceWin + animate. Hmm, forceWin just sets board; need to render. Let me make 'p' set a tile to 2048 and re-render that cell, then checkGameState. Actually checkGameState reads board. Let me:

In onKey, after preventDefault, before handleMove: 
```js
if (e.key === 'p') { 
  // place 2048 in first empty
  for (let r=0;r<SIZE;r++) for (let c=0;c<SIZE;c++) if (!board[r][c]) { board[r][c]={value:2048,id:nextId(),r,c}; renderTile({value:2048,id:board[r][c].id,r,c},'new'); checkGameState(); return; }
}
```

But board is in closure; renderTile uses board? renderTile just renders given tile. OK.

Let me add this debug, reload, press p, inspect (should show win overlay "2048"), then press arrow to continue (overlay hides), then remove debug.

Actually, the win overlay autoHide? No, win overlay doesn't auto-hide (I only auto-hide 'hint'). Win stays until next move hides it (handleMove hides if won && overlay.classList contains 'win'). Let me test: press p → win overlay. Press ArrowLeft → handleMove hides win overlay, does a move. Good, "continue after win."

Let me also test game over: I can force a full board with no merges. Hard quickly. Let me test isGameOver by a debug 'g' that fills board with no-adjacent-equal? Let me just set a checkerboard 2/4 pattern that's full and non-mergeable:
```
for (let r=0;r<SIZE;r++) for(let c=0;c<SIZE;c++) board[r][c]={value: (r+c)%2?2:4, id:nextId()};
```
Then render all and checkGameState → should show Game Over. But existing tiles need removal first. This is getting complex. 

Given time, let me at least verify win (most important unverified). Game over I'll trust the logic (it's straightforward and uses moveBoard.moved which I've verified works for detecting moves).

Actually, let me verify game over too since it's a required behavior. Let me make a debug 'g' that resets to a full non-mergeable board and renders. I'll write a renderBoard helper inline.

Hmm, this is a lot of debug. Let me be efficient: write one debug function `debugState(type)` triggered by 'p' (win) and 'o' (gameover), test both, then remove.

Let me edit the file to add debug, but I must remember to remove it. Let me add it clearly marked and remove after.

Actually, given the risk of leaving debug code and time pressure, let me do a lighter verification: I'm confident in win/gameover logic. The REQUIRED verification items from the task:
1. Play sequence: tiles slide, valid move adds a tile, merge updates score. ✓ DONE (verified: 2→4 score 4, spawn added 2, count correct).
2. New Game resets to two tiles, score reset. ✓ DONE.

The task says "Verify: Play a short sequence... Use New Game...". I've done both. Win message and Game Over are "required behavior" to implement (which I did) but the verify checklist focuses on the two items above. I have time for one more check—let me quickly verify the win overlay appears, since it's a key required feature and easy to break.

Let me add a quick temporary debug key, test win, then remove. I'll be careful to remove it.

Let me edit to add debug 'p'.

Let me look at the onKey function to add debug.

Now let me add a temporary debug key 'p' to force a win, and 'o' to force game over. Let me insert it into onKey before the `if (!dir) return;` check, or handle debug keys separately. I'll add debug handling at the start of onKey.

Let me add:
```js
    // DEBUG
    if (e.key === 'p') {
      e.preventDefault();
      for (let r = 0; r < SIZE; r++) for (let c = 0; c < SIZE; c++) {
        if (!board[r][c]) { board[r][c] = { value: 2048, id: nextId() }; renderTile({ value: 2048, id: board[r][c].id, r, c }, 'new'); checkGameState(); return; }
      }
    }
```

And for game over, 'o':
```js
    if (e.key === 'o') {
      e.preventDefault();
      // fill with non-mergeable pattern
      document.querySelectorAll('.tile').forEach(el => el.remove()); tiles.clear();
      for (let r=0;r<SIZE;r++) for(let c=0;c<SIZE;c++){ board[r][c]={value:(r+c)%2?2:4,id:nextId()}; renderTile({value:board[r][c].value,id:board[r][c].id,r,c}); }
      checkGameState();
      return;
    }
```

Let me add both. I'll edit the onKey function.

Fresh board with two 2's. Now press 'p' to force a win.

Win overlay shows "2 0 4 8!" and "Keep going! (tiles keep merging above 2048)". The board has tiles 2, 2, 2048. `won=true`. Now let me verify "continue after a win" — pressing an arrow should dismiss the win overlay and allow play. Let me press ArrowLeft.

Wait, there's a subtlety: handleMove hides the win overlay if `won && overlayEl.classList.contains('win')`. After pressing 'p', won=true and overlay shows 'win'. When I press ArrowLeft, handleMove: `if (won && overlayEl.classList.contains('win')) hideOverlay();` then proceeds. The board: tiles at (positions of the two initial 2's plus the 2048). ArrowLeft will slide/merge. Let me check if it moves. The two 2's might merge → score +4, or just slide. The 2048 stays. Let me press ArrowLeft and inspect — win overlay should be gone and a move executed (with spawn if moved).

Let me press ArrowLeft.

After ArrowLeft: win overlay dismissed (no "2048!" text in DOM anymore). Score went to 4 (the two 2's merged into 4), best updated to 4. Board now has tiles: 2, 2, 2048, 4, 2 (5 tiles). The win overlay is gone (good, continue works). 

So continue-after-win works. Now let me test game over via 'o'. But note: the board is currently in a "won" state (won=true) with a 2048 tile. Pressing 'o' will fill with checkerboard and call checkGameState. hasWon() is still true (board... no, I'm replacing board with checkerboard 2/4, no 2048). But `won` flag is still true from before, and checkGameState: `if (!won && hasWon())` — won is true so skips win. `else if (isGameOver())` → checkerboard full, no merges → isGameOver true → gameOver=true, shows Game Over overlay with New Game button. Good. But the `won` flag persists; that's fine for game over.

Actually wait, there's a subtle issue: after winning then continuing, `won` stays true. If later the board fills with no moves, isGameOver → show Game Over. Fine. But if I never reset, won stays true. New Game resets won=false. Good.

Let me press 'o' to test game over.

Game Over overlay shows "Game Over" with a "New Game" button. The board is full with checkerboard 2/4 (no adjacent equals), so game over correctly detected. 

Now let me verify that after Game Over, arrow keys don't do anything (canMove false, gameOver true). And the New Game button (overlay) resets. Let me press an arrow — should be ignored. Then click the overlay's New Game button.

The overlay New Game button: it's centered in the overlay (the overlay has flex column align-items center). Position: overlay covers the board (448x448). The button is centered. Board center: x = PADDING(15) + ... boardEl is centered on page. Board spans x: (1200-448)/2 = 376 to 824. Center x = 600. Button centered at x=600. y: board spans y from header height+... Let me compute board top. wrapper padding-top 26, header ~72 tall, gap 16 → board top ≈ 26+72+16 = 114. Board spans y 114 to 114+448=562. Center y = 114+224 = 338. Button centered at y=338. Click x=600, y=338.

Let me first press an arrow to confirm it's ignored (DOM shouldn't change), then click the overlay New Game button.

Actually, pressing an arrow when gameOver: handleMove returns early (gameOver true). So no change. Let me verify quickly then click New Game (overlay).

Let me click at x=600, y=338 to hit the overlay's New Game button.

Actually, let me first press ArrowLeft to confirm ignored, then click.

Actually, to save time, let me just click the overlay New Game button directly. If it resets, game over verification is complete.

But I also want to confirm arrows are ignored during game over (part of "prevent input corruption"). Let me press one arrow, then click. Two steps. Let me just click the button.

Click x=600 y=338.

The click didn't reset — the DOM is unchanged. The overlay button click likely didn't register because the overlay's New Game button may not be at exactly 600,338, OR the click hit the overlay background (pointer-events) rather than the button, OR the button's click handler wasn't attached properly.

Wait, the overlay div has class 'overlay gameover'. The button is inside. The overlay has z-index 10 and covers the board. But the board has the hint/win overlay... no, this is gameover overlay. The click at 600,338 — is that within the board? Board x: 376..824, y: 114..562. (600,338) is within. The overlay covers the board. The button is centered in overlay. So click should hit the button.

But the DOM didn't change and no reset. Possibilities:
1. The click hit the overlay's sub div or title area, not the button. The button is below the "Game Over" title. The overlay has flex column with gap 14. Title "Game Over" (font 48px) then sub containing the button. The button is in the "sub" div. So vertically: title (~50px tall) then gap 14 then button. Total height ~64+14+button(44) ~120. Centered in 448 → starts around y=338-60=278. So button around y 330..375. x=600 should be within button (centered, button width ~150 → x 525..675). So (600,338) should be on the button.

2. Maybe the click handler wasn't attached. showOverlay for gameover: `const b = overlayEl.querySelector('#ovNew'); if (b) b.addEventListener('click', resetGame);`. The button id is 'ovNew', class 'button'. Should attach.

3. Maybe the click didn't fire because overlay has the board behind and board tiles have pointer-events:none, overlay is on top. The click should reach the button.

Hmm, but maybe the issue: the overlay's `pointer-events`? I didn't set pointer-events on overlay. Default is auto. The overlay is a positioned (absolute) div covering board. It should receive the click. Unless board has higher z-index? board is the parent (position relative). Overlay is child with z-index 10. Tiles have z-index log2(value) (1-12). So overlay (z 10) is above tiles. The button should get the click.

Wait — maybe the click coordinate system is wrong, or the click landed on the board's grid/tile not the overlay. Let me reconsider: is the overlay actually covering the board? overlayEl className='overlay gameover'. CSS .overlay { position:absolute; inset:0; ... z-index:10; }. boardEl is position:relative. So overlay covers boardEl fully. (600,338) within board → should hit overlay → button.

Let me reconsider: maybe resetGame ran but the board didn't re-render because... resetGame clears tiles, spawns 2, renders. The DOM should show 2 tiles and score 0. But it shows Game Over still and old tiles. So resetGame did NOT run.

Possibility: the click missed the button (hit overlay background, which has no handler → nothing). Or hit the "sub" div background. Either way no handler → no reset. So my coordinate estimation was off, OR the button region is different.

Let me reconsider the button size. Button: "New Game" text at 20px font, padding 12px 26px. Text "New Game" ~ 100px wide (4 chars "New" ~60 + gap + "Game" ~50... actually "New Game" two words). At 20px bold, ~120px. Button width ~ 120 + 52 (padding) = 172. So x 514..686. y as computed. (600,338) should be inside.

Hmm wait, maybe the issue is the click event isn't reaching because the iframe sandbox blocks? No, clicks work (New Game header button—did I test it? The first New Game test via click at (810,60) DID work and reset). So clicks work in the header region. The board region click should also work.

Wait, actually, did the header New Game click really work, or did I misattribute? Let me recall: before that click, DOM showed 3 tiles "2 4 2" score 4. After clicking (810,60), DOM showed "2 2" score 0 BEST 4. Yes that was the header New Game button and it reset. So clicks work.

So the gameover overlay button click at (600,338) should work but didn't. Maybe my coordinate is off. Let me reconsider the board's vertical position.

The board top: wrapper padding-top 26px. Header: the header contains title "2048" (62px) and score-panel. The title font 62px, line-height ~1. The score-box: label 12 + value 30 + padding 10*2 = 72. So header min-height ~72. With padding-top 26 → header bottom ≈ 26+72 = 98. Then `.wrapper` gap 16 → board top = 98+16 = 114. Board height 448 → board bottom = 562. Center = 338. OK so y=338 is board center.

But the overlay button: the overlay content is centered in the 448 board. "Game Over" title (48px) + sub(button). The button vertical center ~ around board center 338. So button y ~338. Should be hit.

Hmm. Unless the click at (600,338) hit the board's grid cells which have... no handler. Or hit a tile (pointer-events:none). 

Wait—maybe the overlay is NOT on top because of stacking. Let me check: tiles z-index = log2(value). For value 4 → z=2, value 2→z=1. So tiles z-index 1-2. Overlay z-index 10. Overlay should be on top. So click hits overlay.

Unless... the overlay `inset:0` makes it 448x448 at board origin (x376..824, y114..562). (600,338) inside. Button centered. Should work.

Let me try clicking again at a slightly different spot, maybe the button is lower. Or maybe the problem: after 'o' forced game over, `won` was already true (from 'p' win + continue). checkGameState: `if (!won && hasWon())` → won true, skip. `else if (isGameOver())` → isGameOver()? The checkerboard board is full and non-mergeable → isGameOver returns true. But wait, isGameOver calls moveBoard(board,d) for each dir and checks .moved. moveBoard on checkerboard: each line is [4,2,4,2] (or [2,4,2,4] etc). slideLine: compacted all non-zero, no two equal adjacent? [4,2,4,2]: 4≠2, 2≠4, 4≠2 → no merges, out = same order, moved = out[i].id !== cells[i].id? out tiles are the same objects (compacted[i].tile reused), so out[i] = compacted[i].tile which equals cells[i].tile (same object) → id same → not moved. So moved false. All directions → game over true. 

So overlay gameover shown with button. Click should reset. Let me try clicking a bit lower/right. Maybe the button rendered lower. Let me click around (600, 360) — slightly below center. Or maybe the title "Game Over" pushed button down. Title 48px + sub. Center of overlay content: total content height = title(~50) + gap(14) + button(~44) = ~108. Centered in 448 → top of content = (448-108)/2 + boardTop = 170 + 114 = 284. Title 284..334. gap. Button 348..392. So button center y ~ 370! I underestimated. Let me click y=370, x=600.

So my click at y=338 hit the title/gap area, not the button. Let me click y=370.

Let me also remove the debug code after. But first let me verify the gameover reset with a correct click.

Click x=600, y=370.

The gameover New Game button reset the board: score 0, two 2 tiles, best persists at 4. Game over reset works.

Now let me remove the temporary debug code ('p' and 'o' keys). Let me edit out the debug block.

Let me also double-check the hint overlay auto-hide and that hint overlay doesn't block gameplay — it auto-hides after 900ms. But there's a concern: checkGameState shows a 'hint' overlay after every normal move that doesn't win/lose. That hint covers the board for 900ms saying "Press arrow keys to slide". That might be annoying during play but it's harmless. Actually, it could be slightly annoying to always pop up after each move. But it auto-hides. Hmm, during a normal game, after every move a hint overlay appears for 900ms then hides. That's a bit much visually but acceptable. Actually, it might obscure tiles right when player wants to see spawn position. Let me reconsider — maybe remove the hint entirely to avoid clutter, OR only show it on first move. 

Given the verification focus, the hint is a nice-to-have. But it appearing after EVERY move is ugly and could obscure. Let me make the hint appear only on the very first move (or remove it). Actually, simpler: remove the hint branch — only show overlays for win and game over. Let me change checkGameState to not show hint. That's cleaner and more polished.

Let me update checkGameState:
```js
function checkGameState() {
  if (!won && hasWon()) { won = true; showOverlay('win', '2 0 4 8!', 'Keep going! tiles keep merging above 2048'); }
  else if (isGameOver()) { gameOver = true; showOverlay('gameover', 'Game Over', '<button class="button" id="ovNew">New Game</button>'); }
}
```
Remove the hint branch. And remove the autoHide param usage.

Let me edit. Also remove debug keys.

Let me do both edits.

The first edit (debug removal) succeeded but the second didn't match because I put both in one call? No, I made two separate edit objects in the array. Wait, the error says "Could not find edits[1]". I had two entries in the edits array? No—I made one `edit` call with `edits` array containing two objects? Let me re-read. I made a single edit call with edits array of... actually I wrote two top-level things. Let me re-issue.

Wait, my call had `edits` (the debug oldText→newText) and then another object `{"oldText": ...checkGameState...}`. Actually I think I mistakenly structured it. The tool got edits as an array with two elements but the first used wrong format. Let me redo both edits cleanly as two separate edit calls.

Let me first verify current state of the debug block and checkGameState.

I keep making format mistakes. The tool wants `edits` as an array of {oldText, newText}. But I also passed top-level `newText` and `oldText`? No. The error says edits[0].newText missing — because my first array element only had oldText (I put newText at top level by mistake). Let me be careful.

I need to remove the debug block. Let me do a single clean edit with edits array containing one object {oldText, newText}.

The oldText (debug block + dir line):
```
    if (e.key === 'p') { e.preventDefault();
      for (let r=0;r<SIZE;r++)for(let c=0;c<SIZE;c++) if(!board[r][c]){ board[r][c]={value:2048,id:nextId()}; renderTile({value:2048,id:board[r][c].id,r,c},'new'); checkGameState(); return; } }
    if (e.key === 'o') { e.preventDefault();
      document.querySelectorAll('.tile').forEach(el=>el.remove()); tiles.clear();
      for(let r=0;r<SIZE;r++)for(let c=0;c<SIZE;c++){board[r][c]={value:(r+c)%2?2:4,id:nextId()};renderTile({value:board[r][c].value,id:board[r][c].id,r,c});}
      checkGameState(); return; }
    const dir = keys[e.key];
    if (!dir) return;
```
newText:
```
    const dir = keys[e.key];
    if (!dir) return;
```

Let me do this as a single edit call with edits array of one object.

Now remove the hint branch in checkGameState. Let me edit.

Now there's a leftover `beforeIds` variable in handleMove that's unused. Let me check and remove it for cleanliness. Let me read handleMove.

Also `showOverlay` still has the `autoHide` param handling which is now never used (fine, harmless). And the 'hint' class is never used. Fine.

Let me check handleMove for the unused beforeIds.

Let me look at handleMove to remove the unused `beforeIds`.

Now let me also reconsider the score flash CSS - the .scored class is referenced but is there a CSS animation for it? I add `.scored` to scoreEl (the .value). I didn't define a `.scored` animation in CSS. Let me add one for polish. Let me add CSS:
```
.value.scored { animation: scorePulse .18s ease; }
@keyframes scorePulse { 0% { color:#f9f6f2; } 50% { color:#fff58f; } 100% { color:#f9f6f2; } }
```
scoreEl is `document.getElementById('score')` which is the `.value`. Adding 'scored' class. Let me add the CSS rule. It's a nice touch but not required. Let me add it for polish since I have time.

Actually, let me focus on verifying the final working state. Let me reload and do a clean end-to-end test of normal play: New Game already done; let me do a few arrow moves and confirm clean merge/score, then verify no errors after my edits.

Let me reload and play.

Also I should double check: after removing debug, pressing 'p'/'o' does nothing (returns early since not in keys). Good.

Let me reload and run a quick sequence then inspect console for errors.

Let me add the score pulse CSS first for polish, then reload.

I accidentally included `edits[1]` referencing .footer. Let me just do one edit.

It seems the tool thinks I'm providing multiple edits. I only provided one edit object. Oh wait — the error mentions edits[0] and edits[1] overlap but I only gave one. Maybe it parsed my newText/oldText differently. Actually the error is odd. Let me reconsider: maybe the issue is that I previously had `edits2` confusion. Let me just carefully do the edit with a single clean object.

Actually the error "edits[0] and edits[1] overlap" suggests it saw two edits. Perhaps my JSON had an issue. Let me retry with a minimal, clean call.

I keep messing up the JSON — the `edits` is being treated as a string, not an array. The issue is the tool signature: `edits` is an array of objects. My calls are malformed. Let me look very carefully at the required format:

```
"edits": [
  {
    "oldText": "...",
    "newText": "..."
  }
]
```

I need to provide a proper JSON array. My earlier successful edits used this format correctly. The problem now is I'm passing `edits` as a string or with syntax issues. Let me write a clean call.

Actually, the score flash is a minor polish. Given time pressure (was 15 min, ~5 left, now likely <5), let me skip this cosmetic edit and just verify the final state works after removing debug code. The core functionality is what matters.

Let me reload and do a final verification play-through, checking console for errors.