feat: store color directly in Cell (reality layer)

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).
126 tests, 0 failures

src/render/vulkan.rs

@@ -207,7 +207,6 @@ impl VulkanRenderer {
                     ('?', [80, 80, 80, 255], bg_default)
                 } else {
                     let cell = grid.get(wx, wy);
-                    let mat = reg.get(cell.material);
                     if cell.is_empty() {
                         (' ', [10, 10, 15, 255], bg_default)
                     } else {
@@ -215,10 +214,10 @@ impl VulkanRenderer {
                             let r = 200u8.saturating_add(cell.variant / 2);
                             [r, 60, 20, 255]
                         } else {
-                            [mat.color_fg.0, mat.color_fg.1, mat.color_fg.2, 255]
+                            [cell.fg[0], cell.fg[1], cell.fg[2], 255]
                         };
-                        let bg = [mat.color_bg.0, mat.color_bg.1, mat.color_bg.2, 255];
-                        (mat.display_char, fg, bg)
+                        let bg = [cell.bg[0], cell.bg[1], cell.bg[2], 255];
+                        (cell.material.display_char(), fg, bg)
                     }
                 };