Each cell now stores its own fg/bg color inline, instead of looking
up from MaterialRegistry every frame.
Cell struct:
- fg: [u8; 3] — foreground color (for rendering)
- bg: [u8; 3] — background color (for rendering)
- temp: f32 — temperature (already there)
- material: MaterialId — for physics property lookup
- variant: u8 — visual/behavioral variant
- updated_this_tick: bool — CA flag
Colors are copied from MaterialRegistry at Cell::new() time.
This allows per-cell color variation in the future (water depth
shading, lava gradient, damaged materials, etc.) without registry
changes.
MaterialRegistry still stores physics properties (density, solid,
liquid, flammable, etc.) — looked up only during CA/physics steps,
not during rendering.
Renderers (terminal, ascii, graphics) updated to use cell.fg/cell.bg
directly — no more registry lookup in render path.
Cell size: ~16 bytes (was ~12). 250x250 grid = 1MB (was 750KB).
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Emil
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