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/graphics.rs

@@ -344,7 +344,6 @@ impl GraphicsRenderer {
                     [40, 40, 40, 255]
                 } else {
                     let cell = grid.get(wx, wy);
-                    let mat = reg.get(cell.material);
                     if cell.is_empty() {
                         bg_default
                     } else {
@@ -352,7 +351,7 @@ impl GraphicsRenderer {
                             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]
                         }
                     }
                 };

src/render/terminal.rs

@@ -86,26 +86,18 @@ impl Renderer for TerminalRenderer {
                     continue;
                 }
                 let cell = grid.get(wx, wy);
-                let mat = reg.get(cell.material);
                 let idx = dy * self.width + dx;
                 if cell.is_empty() {
-                    frame[idx] = (' ', mat.color_fg, mat.color_bg);
+                    frame[idx] = (' ', (cell.fg[0], cell.fg[1], cell.fg[2]), (cell.bg[0], cell.bg[1], cell.bg[2]));
                 } else {
-                    let ch = if cell.material == MaterialId::Lava {
-                        if cell.variant % 2 == 0 { '#' } else { '#' }
-                    } else if cell.material == MaterialId::Water {
-                        if cell.variant % 3 == 0 { '~' } else { '~' }
-                    } else {
-                        mat.display_char
-                    };
+                    let ch = cell.material.display_char();
                     let fg = if cell.material == MaterialId::Lava {
-                        let flicker = cell.variant;
-                        let r = 200u8.saturating_add(flicker / 2);
+                        let r = 200u8.saturating_add(cell.variant / 2);
                         (r, 60, 20)
                     } else {
-                        mat.color_fg
+                        (cell.fg[0], cell.fg[1], cell.fg[2])
                     };
-                    frame[idx] = (ch, fg, mat.color_bg);
+                    frame[idx] = (ch, fg, (cell.bg[0], cell.bg[1], cell.bg[2]));
                 }
             }
         }

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)
                     }
                 };
 

src/world/cell.rs

@@ -67,6 +67,8 @@ pub struct Cell {
     pub temp: f32,
     pub updated_this_tick: bool,
     pub variant: u8,
+    pub fg: [u8; 3],
+    pub bg: [u8; 3],
 }
 
 impl Cell {
@@ -76,10 +78,14 @@ impl Cell {
             temp: 20.0,
             updated_this_tick: false,
             variant: 0,
+            fg: [15, 15, 20],
+            bg: [10, 10, 15],
         }
     }
 
     pub fn new(material: MaterialId) -> Self {
+        let reg = MaterialRegistry::instance();
+        let mat = reg.get(material);
         let temp = match material {
             MaterialId::Lava => 1500.0,
             MaterialId::Fire => 800.0,
@@ -92,6 +98,8 @@ impl Cell {
             temp,
             updated_this_tick: false,
             variant: rand_u8(),
+            fg: [mat.color_fg.0, mat.color_fg.1, mat.color_fg.2],
+            bg: [mat.color_bg.0, mat.color_bg.1, mat.color_bg.2],
         }
     }