The user wants me to create a single-page web app that implements a Gray-Scott reaction-diffusion simulation. This is a classic Turing pattern formation model where two chemicals (A and B) react and diffuse at different rates, creating organic patterns like spots, stripes, and labyrinthine structures.

Key requirements:
1. Single self-contained index.html file
2. CSS in <style> tag, JS in <script> tag - all inline
3. No external resources, CDNs, or network requests
4. Canvas-based visualization
5. Real-time animation with organic Turing patterns
6. Mouse interaction to seed new growth

The Gray-Scott model uses these equations:
- dA/dt = D_A * ∇²A - AB² + f(1-A)
- dB/dt = D_B * ∇²B + AB² - (k+f)B

Where:
- A and B are concentrations of the two chemicals
- D_A and D_B are diffusion rates
- f is the feed rate
- k is the kill rate
- ∇² is the Laplacian (diffusion operator)

I'll need to:
1. Set up a canvas for rendering
2. Implement the reaction-diffusion algorithm using arrays for chemical concentrations
3. Use a discrete Laplacian for the diffusion calculation
4. Add mouse interaction to add/seed chemicals
5. Animate the simulation in real-time
6. Visualize the patterns with nice colors

Let me write this comprehensive implementation.

I've created a complete Gray-Scott reaction-diffusion simulation in a single self-contained HTML file. Let me verify it was written correctly.

The file was written successfully. Let me provide a summary of what the simulation includes.