We need to write an index.html single file with CSS and JS implementing Gray-Scott reaction-diffusion simulation with mouse seeding. No external resources. We'll create a canvas, use WebGL or 2D canvas? For simplicity 2D canvas with per-pixel arrays.

Implementation: We'll use Float32Array for U and V. Simulation step: use discretized equations with diffusion and reaction. Use typical parameters: Du=0.16, Dv=0.08, F=0.035, k=0.065.

We will create a simple UI: canvas full screen. On mouse move with button pressed, add feed or modify values near mouse.

We'll implement time loop via requestAnimationFrame. We'll compute on each frame, and draw to canvas by mapping V or U to grayscale.

We need to maintain arrays of size width*height. On each frame compute new U,V. We'll use temporary arrays or double buffer.

Will use a simple algorithm: For each cell, compute Laplacian with 4-neighbor approximations.

Because of performance, use typed arrays and minimal overhead.

Also we need to handle resizing.

Let's craft index.html.

We'll need to handle mouse events: on mousedown or mousemove with button pressed, add random perturbation: e.g., set U=1, V=0 at a radius around mouse.

Ok.

Let's produce the file. We'll use <style> for basic fullscreen and canvas styling. We'll embed script.

We need to run simulation quickly.

Let's write code.

After writing, we need to write file to path.

Let's produce using write tool.

We wrote a file. Need to verify that this matches the request: single-page web app, with Gray-Scott reaction diffusion, mouse seeding. We used inline CSS, JS, all in one file. No external resources. Good.

Maybe add a bit more polish? But question satisfied. Should we add some comments? Not necessary.

We should respond with final.