We need respond by building file index.html in current dir. Need inspect? Need create self-contained HTML. Need implement 2048 with arrow key sliding/merging, smooth animations, score, win/lose. We need ensure no external resources. Need likely use write. We should create index.html. Need maybe include CSS animations. Need robust game logic. Let's craft.

Need think through implementation details. Single-page web app, playable. Use arrow keys tile sliding and merging, smooth animations, score, win and lose states. Need file present. We can write all in one file. We need maybe include restart, new game, maybe mobile swipe. User only asked arrow-key, but can include swipe optional. Need no external resources. Need animations. Need maybe use CSS transitions. Need tile merging animation? 2048 tile sliding and merging smooth. We can implement grid with absolute positioned tiles, JS calculates positions and transitions. Need avoid duplicate animations on merge.

Let's design.

HTML:
- header with title, score panel, best maybe.
- game area 4x4, size maybe 520 CSS, .board with 4x4 cells.
- tiles absolutely positioned based on row/col. Use transform translate(x, y). CSS transition: transform 110ms cubic-bezier. For spawn use fade/scale. For merge maybe flash.
- Need update state after move. We'll compute previous positions and new positions, apply.
- Use "tiles" array? Simpler: each move:
- compute new board and move info:
- old positions: board tiles map by id? We can assign id to each tile.
- For each tile, keep id and value.
- Move: for each direction, create rows/cols, process in order.
- Need tile positions for sliding. Use id.
- Compute for each tile old position (row,col) before move and new position. For tiles not moved, new same.
- Use tile IDs to identify.
- Board state is 2D with objects {id,value}.
- After move, if changed:
- oldPositions = new Map from id to {row,col}
- new board.
- animate from old to new:
- remove old tile elements? Need update DOM with transitions.
- We can maintain DOM elements keyed by id. On move, for each tile id in union old/new, create/update element with class, set style transform. Use transition transform. For old positions, set transform before force reflow then set new transform.
- For new spawned tile, create element with transform at new position and class spawn, then force reframe, then remove spawn, add animate class maybe.
- For merge: value doubled; class maybe merged. Need show merged tile. It's okay if animation shows new value after movement. Could add class merge to tile that merged. CSS keyframes pop.
- For tiles that don't move but maybe merge? no.
- If no move, maybe shake board.
- Score updates.
- Win state: if new value >= 2048 and not already won, show modal. Could allow continue? User asked win state. We'll show "You made 2048! You win!" and maybe "Keep playing" and "New game".
- Lose state: if no moves, show "Game over".
- Keyboard arrows prevent default. Buttons.
- Need accessibility maybe.

Potential issue with CSS transitions and setting positions. We can use requestAnimationFrame.

Simplify: Use DOM as CSS grid with absolute .tile. .tile { position:absolute; width: calc((100%-padding)/4)...; transition: transform .12s ease, opacity .12s; } Need board padding. Easier use fixed board size 520 and cell size 120, gap 14. CSS:
.board { width: min(92vw, 520px); aspect-ratio:1; position:relative; background: #bbada0; border-radius; padding:14px; box-sizing:border-box; }
.tile { position:absolute; left:0; top:0; width: calc((100% - 42px)/4); height:same; transform: translate(xpx, ypx); transition: transform 130ms cubic-bezier...; }
But calc with variable: --cell-size? Could use JS compute positions in px:
cell = (board.clientWidth - 3*gap)/4; x=col*(cell+gap), y=row*(cell+gap). Set style.width/height and transform.
Need after board resize recalc positions. We can use CSS grid? Another approach: .tile position absolute with percentage transform: translate(col*25%, row*25%) and width 25%, height 25%. This is simpler and responsive. But padding? If board is grid with cells, tile width 25%, height 25%, transform translate(col*25%, row*25%). No padding. Board background with grid gaps via box-shadow inset? We can use CSS grid background repeating-linear-gradient? But tile size with gaps via background. Could use board padding 14px and tile size calc((100% - 42px)/4); transform percentages won't include padding. Use JS compute.

Could use CSS grid for cells and tile in cell. Sliding animation less smooth? Could animate left/top with %? We'll use JS px.

Need robust.

Let's write index.html.

Game logic:
- const SIZE=4
- let board, score, best, gameWon, gameOver, active = true.
- board: 2D null or {id,value}.
- nextId=1.
- newGame():
- board = empty
- score=0; won=false; gameOver=false; nextId=1; spawnRandom(); spawnRandom(); render();
- spawnRandom:
- find empty cells; if none return false.
- choose random; value=Math.random()<0.9?2:4; push {id: nextId++, value}
- render:
- maybe clear tileLayer and create elements for each tile. But for animations need not clear. We'll use separate tileLayer.
- renderBoard: tileLayer.innerHTML = ''; for each tile create div class tile valueClass(value)
- renderTile(id,value,row,col, options)
- For initial render, set position and add class spawn.
- updateDomFromPositions(oldPositions, board):
- Need create missing, update existing.
- For each tile in board, ensure tile element exists.
- Set class value.
- For each tile id in oldPositions: if not in new? remove.
- For each tile id in union:
- el.dataset.id
- set width/height based on current board size? We'll calculate in function layoutTile(el,row,col).
- For tiles with old pos and new pos maybe same:
- if new pos same: set transform to new (no transition maybe)
- else: set transform to old first, then requestAnimationFrame set transform new. But if we set old then immediately new without reflow, transition won't run. So:
el.style.transform = oldTransform;
el.offsetHeight; // force reflow
el.style.transform = newTransform;
- If no old pos (spawn):
- set width/height, transform new, class spawn, then requestAnimationFrame remove spawn and add class maybe.
- For merged class:
- el.classList.add('merged'); setTimeout remove.
- Need oldPositions from old board.
- Need compute move.

move(direction):
- if gameOver return.
- oldBoard = deep clone? Need preserve tile ids/values. board is 2D objects. Use cloneBoard = board.map(row=>row.map(tile=> tile ? {...tile}: null)).
- oldPositions = new Map(); board.forEach ...
- result = getNewBoard(board, dir)
- if equal return false.
- oldBoard = clone before move.
- board = result.board
- if result.scoreDelta score += delta
- if result.mergedValues includes id? We need mark merged tiles. In getNewBoard, if merging, mark new tile id merged? But old tile id disappears. We need merged tile id from first tile maybe. Mark {id: target.id, value: newVal}. Or pass mergedIds Set.
- Then renderScore, animate(oldBoard, board, mergedIds)
- if !gameOver check win/lose.
- Need update best localStorage? no external. Use localStorage maybe sandbox. Could include best. But not necessary. Score requested. Best optional. We can use localStorage; if blocked maybe try/catch.
- render DOM after animation.
- Need if no move: add shake class to board; setTimeout remove.
- Need check win after each move if merged tile value >= 2048.
- If win modal shown. Could keep playing after win by Continue. User wants win state; we can show. Maybe if won, gameOver false but modal overlay. If user clicks keep playing, hide modal and keep. If clicked New Game reset. If no move, gameOver true.

Move algorithm:
function moveDirection(dir):
- For rows:
- up: for each col c, collect tiles from r=0..3 with board[r][c], in order.
- Then fill positions:
let pos=0; for each tile:
if pos==0: newBoard[r][c]=tile (same id)
else merge with previous:
newVal=tile.value+prev.value;
prev.value = newVal; prev.id = prev.id? (same)
prev.merged = true?
score += newVal;
mergedIds.add(prev.id)
pos=0
else:
newBoard[r][c] = tile
pos++
But careful if we mutate oldBoard? We clone before, so okay. Need if no previous tile? We'll mutate clone.
- down: collect r=3..0, fill r=3..0
- left: for each row, collect c=0..3, fill c=0..2
- right: collect c=3..0, fill c=3..0
- Need detect changed by comparing board and newBoard including ids. Since clone before, can use JSON.stringify? Objects with id. We can compare. Use sameTile.
- Need for merge mark mergedIds in result. But oldBoard clone has same id.
- Need if no move but board equal.
- Need maybe oldBoard object references? cloneBoard = board.map(row=>row.map(tile=> tile ? {id:tile.id,value:tile.value}: null)) before move. Then move mutates clone. Good.
- Need oldPositions from old clone.

Potential issue: when merging, we mutate tile.value, but tile object in clone. Good.
- Need scoreDelta.

Position layout:
function getMetrics() {
const rect = boardEl.getBoundingClientRect();
const gap = Math.max(8, Math.floor(rect.width*0.026)); maybe 14.
const cell = (rect.width - gap*(SIZE-1))/SIZE;
return {cell, gap}
}
function place(el,row,col) {
el.style.width = cell+'px'; height...
el.style.transform = `translate(${col*(cell+gap)}px, ${row*(cell+gap)}px)`;
}
Need ensure .tile uses left:0; top:0.

Need update all tile sizes on resize. If no animations? We can call renderStatic to clear and recreate. But if user resizes during game, okay.
window.addEventListener('resize', () => { render(); }); But render clears all tiles no animation. Fine.

Need CSS:
- body background, font system, no external.
- .app container.
- .topbar flex.
- .score-card.
- .board background #bbada0; border-radius; box-shadow; overflow:hidden; touch-action:none.
- .tile backgrounds by value. Could define classes: .tile-2, .tile-4 etc. Need many classes up to maybe 2048/4096. Could generate CSS for values 2,4,8,16,32,64,128,256,512,1024,2048,4096,8192. Text color dark except 1024+.
- .tile.font-small for >=1024 maybe.
- .tile.spawn { animation: pop .16s ease-out; }
- .tile.merged { animation: merge .22s ease-out; }
But if element has spawn and merged? Spawn for new tile, merge class for merged tile. Could classList add both.
- .tile { display:flex; align-items:center; justify-content:center; font-weight:800; font-size:clamp?; user-select:none; }
- .tile::after? no.
- .modal overlay hidden. .modal.show display flex.
- .modal card.
- .controls.

Need maybe include instructions.

Need JS after HTML. Use strict.

Need maybe include "Use arrow keys to move tiles".

Need implement accessible buttons.

Potential issue with score update and DOM:
- renderScore sets scoreEl.textContent.
- best update if score > best. localStorage.
- On new game, reset.

Need maybe include "win" state but if win modal covers board. Continue button. New game button.
- showWin():
gameWon=true; showModal('You made 2048!', 'You win!', 'Keep Playing', 'New Game')
- If user continues hide modal; active true. If lose, disable controls? The new game button remains.
- Buttons:
- newGameBtn click newGame
- modalPrimary/secondary.
- Could include "New Game" button in header.

Need ensure no external resources. No fonts.

Potential issue with tile class names for values. We'll generate function valueClass(v) return v>=2048 ? 'tile-2048-plus' maybe.
CSS:
.tile-2 { background:#eee4da; color:#776e65; }
...
.tile-2048-plus { background:#3c3a32; color:#f9f6f2; }
.font-small for >=1024.
Need "tile-2048" maybe.

Need code.

Let's create.

Need verify file maybe run some checks. Need inspect maybe there are bugs. Let's read? Need maybe use bash to check syntax. We can run node? Since script in HTML not module. Could use node --check? It checks JS? Use node --check index.html maybe syntax parse? It may fail because HTML. Could extract script. Better use python? Need maybe simple grep. But we need ensure code works. Let's inspect relevant. Use read.

Need read lower.

Need identify bugs. render function references oldTileIds before declaration? JS let oldTileIds = new Set() is after function declarations, but render not called before declaration except newGame calls render after declaration? Actually newGame function defined after oldTileIds declaration? Let's check order: function render() before let oldTileIds. render closure references oldTileIds; oldTileIds declared after render but before newGame. newGame called at bottom after declaration, so okay.

Potential bug in animateMove for merged tile: mergedIds has id of line[index - 1], which is tile object. But we mutate line[index -1].value; id same. Good. But for multiple merges in same line? In 2048, [2,2,2,2] -> [4,4,null,null], first merge line[0] id1 value4, second merge line[2] value4 with line[1] value2, not line[0] because index > 0 uses line[index -1].value (line[1]) not line[index -2]. Actually correct? For [2,2,2,2], index 0 place tile0 value2, index1 nextTile = tile1 value4, score +=4, line[0].value=4, mergedIds add line[0].id. index2 nextTile = tile2 value + line[1].value (2) =4, score +=4, line[1].value=4. Good. So mergedIds includes tile0 and tile1. But tile1 remains in target with value4. It is also merged. Good.
Potential issue for [2,2,4] -> [4,4], index1 line[0].value=4, index2 nextTile tile2 value+line[1].value (2) =6? Wait line[index-1] is tile1, not line[index-2]. In 2048, after merging tile1 into tile0, the next tile should merge with the newly formed tile (line[0]) if equal, not with original tile1. But original algorithm in JS 2048: if (line[i-1] === 0) continue; if (line[i-1] === line[i]) { line[i-1] *= 2; line[i] = 0; score += line[i-1]; } This merges with previous non-zero, which after merge is line[i-1] (same as line[i-2] after value update). But our code uses line[index -1].value for nextTile, where line[index-1] is current tile before merging, not previous tile. For index2, line[1] still value 2, so nextTile=6. Wrong. Need fix fillLine. It should use previousTile = line[index -1], but after merge previousTile value updated. However line[index-1] is current tile, not previous. Need use `const previousTile = line[index - 1];` before? For index > 0, previousTile should be line[index -1], but after previous iteration line[index-1] has been mutated if index-1 was a merge. In [2,2,4], line[1] was original tile1 value2, line[0] mutated. We need merge nextTile with line[index - 2]? Let's think. At index=1, previousTile = line[0], target tile. At index=2, previousTile = line[1], but line[1] is empty/null after merge? In our fill, we don't null it. We should set line[index-1] = null after merging? Typical algorithm sets line[i] = 0. If we set line[index-1] = null after index1, then index2 previousTile = line[index-1] is null, skip. Need fillLine skip nulls. Better implement standard:

function fillLine(target, direction, line) {
 const mergedIds = new Set();
 let writeIndex=0;
 for (const tile of line) {
   if (!tile) continue;
   const previous = line[writeIndex - 1];
   if (previous && previous.value === tile.value) {
      previous.value += tile.value;
      score += previous.value;
      mergedIds.add(previous.id);
      line[writeIndex - 1] = previous;
      // maybe set line[writeIndex] = null? Not necessary for previous pointer? But if previous = line[writeIndex -1], next tile sees previous = line[writeIndex -1] = previous (good). But current tile at line[writeIndex] remains original tile; if we don't null, next iteration previous remains line[writeIndex] (previous merged) okay. But if we continue, line[writeIndex] is previous. Actually writeIndex doesn't increment. Good. But if tile has same value and previous is current? Let's test [2,2,2]:
      writeIndex=0; tile0 previous none; place line0 tile0. writeIndex=1.
      tile1 previous=line0 value2 equal; previous.value=4, score 4, line0 remains; writeIndex stays 1.
      tile2 previous=line0 value4 not equal; place tile2 at line1; writeIndex=2.
   } else {
      line[writeIndex] = tile; writeIndex++;
   }
 }
 for i writeIndex..3 target null.
 return mergedIds.
}
This is standard. But note line[writeIndex] when writing tile after merge may overwrite original tile1. That's okay. Need not mutate current tile to nextTile object. This preserves id of tile0 for merged and id of tile2. For [2,2,4,4]:
- tile0 writeIndex0
- tile1 prev line0 equal -> prev.value4 score4
- tile2 prev line0 value4 not equal -> line[1]=tile2 writeIndex2
- tile3 prev line1 value4 equal -> line1.value8 score8 mergedIds add line1.id (tile2). Good.
Need implement.

Potential issue with animation of merged tile id: For [2,2,4,4], mergedIds includes tile0 and tile2. Good.

Need edit fillLine. Use edit.

Need more bugs.

Potential bug in win/lose check: In move, after board = newBoard, update score, animate, then if no empty and no moves lose. But if board has 2048 and no empty, lose wins? Current condition checks lose first then win. If board filled and has 2048, lose fires and modal game over. But win should show if tile >= 2048. Usually if you reach 2048 on last move, win. Need reorder to check win first. Also if board filled and has 2048 but no moves? There is a 2048 tile maybe no moves, but win state should show. User asked win state. Should show win. So reorder. Also if lose, won false. Let's edit.

Potential bug: `boardEl.addEventListener("keydown", handleKeydown);` board isn't focusable maybe keydown on board only works if board focused. document also handles. Good.
Potential bug: `document.addEventListener("keydown", handleKeydown);` event keydown on body, prevent default, good.
Potential bug: `tileLayer.innerHTML = "";` then ensureTile uses querySelector. But oldTileIds still contains ids; render removes. Good.
Potential bug: `render` after newGame: oldTileIds is new Set, tileLayer cleared. Good.
Potential bug: `animateMove`: `const newPos = board.flatMap(...).find(Boolean);` This creates array with nulls and objects. Works. But if board has a tile with id 0? IDs start 1. Good.
Potential bug: `el.className = ...` resets class; if tile had spawn animation and then merged? Fine.
Potential bug: For existing tile moved, we set className includes merged maybe. If tile also had spawn? no.
Potential bug: If tile moves and gets merged, class merged animation starts. Good.
Potential bug: If tile moves but no merge, we don't remove any `merged` class because className reset. Good.
Potential bug: If tile moves but oldPos and newPos same? In a valid move, some tiles may not move and no merge. We set oldPos style, force reflow, then new style same. It won't animate but okay. But because className reset, if it previously had `merged` class from prior animation, removed. Good.
Potential bug: If tile moves and oldPos/newPos same? no.
Potential bug: If board is full and no moves, lose. If board has empty but no moves? In 2048, if empty, there is always move? Actually if empty and no adjacent equal, you can still move tiles into empty, so hasAnyMove should be true? Let's check algorithm: if empty, hasAnyMove returns true immediately. So okay.
Potential bug: `hasAnyMove` uses optional chaining. Modern browsers okay.
Potential bug: CSS `backdrop-filter` maybe okay.
Potential bug: `modalSecondary` in lose has empty text but still button with empty text; clicking does nothing. Fine. But it has danger style. Could hide. Not issue.
Potential bug: `continuePlaying` after win hides modal but `locked=false`. `won` remains true so no win modal. Good.
Potential bug: `newGame` resets `locked=false` even if lost. Good.
Potential bug: If user presses New Game while modal show, good. If `modalSecondary.textContent` empty, click no op. Good.
Potential bug: If user presses arrow after lose, `lost` true and `locked` true, no move. Good.
Potential bug: If user presses arrow after win but modal hidden by continue, move works. Good.
Potential bug: If user presses arrow while modal visible before continue, locked false, won true but move not prevented except modal overlay. Keydown document still moves behind modal. Since overlay doesn't capture keyboard. We should prevent moves while modal visible. `won` true but move only checks locked/lost. Should add `if (locked || lost || modal.classList.contains("show")) return;` But after win modal, should not move until continue. However if modal visible, keydown still moves. Need edit. Also after win if user presses New Game, newGame. Let's edit move condition.

Need maybe if modal visible but not locked (newGame not called?) modal only visible in win/lose. Good.
Potential issue: `render` uses `oldTileIds` declared after function but before newGame. Fine. But if `window.addEventListener("resize", render);` fires before `oldTileIds` initialized? Event listeners set before newGame. resize event could fire before oldTileIds declaration? JS synchronous: after listener registration, `newGame()` called before user can resize. So okay.
Potential issue: `boardEl.addEventListener("keydown", ...)` plus document. Prevent default on document may prevent focus? okay.
Potential issue: `fillLine` standard algorithm with `line[writeIndex] = tile` when writeIndex maybe >? For right direction, line collected right-to-left, target placed right-to-left. Good.
Potential issue: `collectLine` for right/up uses source[0] row or col. Good.
Potential issue: `sameBoard(previous,newBoard)` compares ids. If move no change, false. If move changes positions but same values and ids? IDs move with tiles, so changed. Good.
Potential issue: Score update before animation. Good.
Potential issue: Win/lose check after animation. If lose after animation, modal overlay appears. Good.
Potential issue: `animateMove` computes oldPositions from previousBoard and newIds from board. It doesn't consider old tile that gets removed? It removes. Good.
Potential issue: `oldPositions` for merged target tile: previousBoard contains tile with id; board contains same id with doubled value. Good.
Potential issue: For a merge, the losing tile id is not in new board and is removed. It had no animation. That's fine.
Potential issue: For a tile that moves and merges, `mergedIds` includes id of previous tile. Good.
Potential issue: For a tile that doesn't move but is merged? impossible.
Potential issue: For a tile that moves to same position but merge? impossible.
Potential issue: If `oldPos` exists and `newPos` exists but `oldPos.r/c` same and tile class merged, we set className merged and style old, force reflow, style new. Merge animation may happen because class changed. Good.
Potential issue: If `oldPos` exists and tile class changed from tile-2 to tile-4 and moved, class reset. Good.
Potential issue: `stylePosition` uses `width` property; if boardEl.clientWidth 0? During initial render maybe board not laid out? But script at end of body after layout? Browser may compute. If width 0, cell weird. But okay.
Potential issue: On mobile, body center; board maybe too big. Fine.
Potential issue: `touch-action: none` on board. Good.
Potential issue: "smooth animations" yes.

Need maybe run a quick JS parser by extracting script. Use node --check? Let's do.

Process substitution issue? Node can't open. Could write temp? Use bash with mktemp and node --check.

Need maybe test game logic with jsdom? no jsdom. Could extract and evaluate functions? Could create a small script with same functions? Maybe not necessary. But let's reason.

Potential bug in `sameBoard` after move: `previous` clone and `newBoard` objects. For no-move, sameBoard true. For move where tile moves into empty, sameBoard false. Good.

Potential bug in `fillLine`: We use `line[writeIndex] = tile` but if writeIndex > current index? writeIndex <= index always. Good.
Potential bug in `fillLine` with [2,2,2]:
- line = [2a,2b,2c,null]
- index0 tile2a writeIndex0 prev none -> line[0]=2a writeIndex1
- index1 tile2b prev line0 value2 -> prev.value=4 score4
- index2 tile2c prev line0 value4 not equal -> line[1]=2c writeIndex2
- target left: [4a,2c,null,null]. Good.
Potential bug in `fillLine` with [2,2,2,2]:
- line0=2a w1
- tile1 merge prev line0=4 score4
- tile2 prev line0=4 !=2 -> line1=2c w2
- tile3 prev line1=2c equal -> line1=4 score8. target [4a,4c]. Good.
Potential bug in `fillLine` with [4,4,2,2]:
- tile0 write
- tile1 merge line0=8 score8
- tile2 prev line0=8 !=2 -> line1=2c
- tile3 prev line1=2c merge line1=4 score12 target [8,4]. Correct.
Potential bug with right direction [2,2,2,2] collected [2d,2c,2b,2a]:
- tiled line0=2d w1
- tilec merge line0=4
- tileb prev line0=4 !=2 -> line1=2b
- tilea prev line1=2b merge line1=4 target right positions: index0 target[3]=4, index1 target[2]=4. Good.
Potential bug with up/down. Good.

Potential bug in `hasAnyMove`: if empty returns true. So lose only when no empty and no adjacent equal. Good.
Potential bug: If board has no empty but no adjacent equal, lose. Good.
Potential bug: If board has 2048 and no moves, win first. Good.
Potential bug: If board has 4096 etc, win. Good.

Need maybe maybe user expects only arrow-key tile sliding and merging; swipe optional okay. Need no external resources. Good.

Need maybe final response concise mention file created. But before final maybe run `ls -l index.html` to show present.