improve: pre-blur density FBO for smooth normal computation

[jayf0x]

Problem

The in-shader Sobel kernel (8 taps) approximates normals but can't fully eliminate specular aliasing at all resolutions.

Proposed change

Add a blurDensity FBO (same dimensions as density). Run two separable Gaussian blur passes (horizontal + vertical, ~5 taps each) once per frame before the display pass. Bind the blurred texture as the source for normal computation only — the raw density FBO still drives color and alpha.

Acceptance criteria

• blurDensity FBO created, same dimensions as density FBO
• Two-pass separable blur runs before display pass
• Normal computation reads from blurred FBO, not raw density
• Color/alpha still driven by raw density FBO
• No regression in existing visual tests

Cost: 2 extra render passes per frame, ~10 texture taps total.

Files likely touched: src/core/shaders.ts, src/core/wgsl-shaders.ts, src/core/simulation.ts