feat(fluid): add Stable Fluids demo on float textures

Velocity runs on a 128x128 grid and dye on 512x512.
Walls are free-slip: shaders clamp coordinates to the field.
Drag pushes the fluid and adds dye. Idle time adds random splats.
Tick the fluid item in ROADMAP.md.

ROADMAP.md

@@ -30,4 +30,5 @@ Open ideas for the playground frontend. Tick a box when done.
   - [x] Fourier epicycles
   - [x] Reaction-diffusion (Gray-Scott) shader (reuse the fractal shader setup)
   - [x] **Lenia** (continuous Game of Life, shader ping-pong like Gray-Scott)
+  - [x] **Fluid (Stable Fluids)** (velocity, pressure and dye shader ping-pong like Lenia)
   - [x] **Ant colony** (ACO on TSP, CPU + Canvas 2D)