We are going to create a single-page web app that runs a Gray-Scott reaction-diffusion simulation.
 The simulation will be displayed on a canvas and will grow organic Turing patterns in real time.
 We will allow the user to seed new growth by clicking or moving the mouse over the canvas.

 The Gray-Scott model is defined by two chemicals U and V and the following equations:
   du/dt = Du * laplacian(u) - u*v^2 + F*(1-u)
   dv/dt = Dv * laplacian(v) + u*v^2 - (F+k)*v

 We'll use a finite difference method to update the concentrations.

 Steps:
 1. Set up an HTML5 canvas and get a 2D context.
 2. Initialize two 2D arrays (for U and V) with initial conditions (usually U=1, V=0 everywhere, with a small square of V=1 in the center).
 3. Define the parameters: Du, Dv, F, k. These parameters will determine the pattern.
 4. Use a double buffering technique (two sets of arrays) to avoid updating the array we are reading from.
 5. In the animation loop, for each cell, compute the laplacian (using a 3x3 kernel) and then update the concentrations.
 6. Draw the current state of the V concentration (or some function of U and V) to the canvas.
 7. Add event listeners for mouse movement and clicks to add V (or disturb the pattern) at the mouse position.

 We'll use a simple color mapping: V concentration mapped to a color (e.g., from black to white or a color gradient).

 Let's choose parameters that produce interesting patterns. Some known parameters:
   - F = 0.035, k = 0.065 -> spots
   - F = 0.025, k = 0.06 -> labyrinth
   - F = 0.04, k = 0.06 -> waves

 We'll set the canvas to full window size and adjust on resize.

 Since we are writing everything in one file, we'll have:
   - HTML structure: a canvas element
   - CSS: style the canvas to fill the window
   - JavaScript: the simulation code

 Let's code accordingly.

I've created a complete Gray-Scott reaction-diffusion simulation that generates organic Turing patterns in real-time. The web app includes:

**Key Features:**
- **Real-time simulation**: Continuously evolves beautiful organic patterns
- **Mouse interaction**: Click and drag to seed new growth areas
- **Parameter controls**: Adjust feed rate (F), kill rate (K), diffusion rates, and seed size
- **Pattern presets**: Pre-configured parameters for different pattern types (spots, labyrinth, waves, maze, coral)
- **Responsive design**: Adapts to window size changes
- **Pause/resume**: Control the simulation flow

**Technical Implementation:**
- Uses Float32Arrays for efficient computation
- Implements the Gray-Scott reaction-diffusion equations
- Real-time rendering with canvas ImageData
- Touch support for mobile devices
- Efficient buffer swapping for smooth animation

**How to Use:**
1. Open `index.html` in a web browser
2. Watch the patterns evolve naturally
3. Click/drag on the canvas to seed new growth
4. Use the controls to adjust parameters and explore different pattern formations

The simulation creates mesmerizing organic patterns that resemble biological growth, chemical reactions, and natural formations - all driven by the mathematical principles of reaction-diffusion systems.