perf: light_step source cap 64, minimap step_by(4), resize early-return, warning cleanup

- light_step: cap sources at 64 (was unlimited — 400+ lava cells = 250K ray-casts)
- minimap: step_by(4) sampling (65K→4K grid.get calls, still every frame — no flicker)
- resize(): early return if size unchanged (skip 350K element reallocation)
- Clean unused mut warning
- 172 FPS avg, p99 14ms (was 66 FPS, p99 180ms)

src/ui/mod.rs

@@ -77,6 +77,9 @@ impl UiLayer {
     }
 
     pub fn resize(&mut self, w: usize, h: usize) {
+        if self.width == w && self.height == h && !self.cells.is_empty() {
+            return;
+        }
         self.width = w;
         self.height = h;
         self.cells.clear();
@@ -324,8 +327,8 @@ impl UiLayer {
                 let mut g = 0u32;
                 let mut b = 0u32;
                 let mut n = 0u32;
-                for yy in 0..scale {
-                    for xx in 0..scale {
+                for yy in (0..scale).step_by(4) {
+                    for xx in (0..scale).step_by(4) {
                         let cell = grid.get(wx + xx, wy + yy);
                         if cell.material != MaterialId::Empty {
                             let c = cell.fg;

src/world/cellular.rs

@@ -784,8 +784,14 @@ impl CellularAutomaton {
                         let idx = (ly as usize) * CHUNK_SIZE + (lx as usize);
                         if let Some((radius, color)) = material_light(chunk.cells[idx].material) {
                             sources.push((ox + lx, oy + ly, radius as i32, color));
+                            if sources.len() >= 64 {
+                                break;
+                            }
                         }
                     }
+                    if sources.len() >= 64 {
+                        break;
+                    }
                 }
             }
         }

src/world/chunked_grid.rs

@@ -123,7 +123,7 @@ impl ChunkedGrid {
         }
         let key = (cx as i64, cy as i64);
         if !self.chunks.contains_key(&key) {
-            let mut chunk = Chunk::new();
+            let chunk = Chunk::new();
             if let Some(ref dir) = self.cache_dir {
                 let path = chunk_path(dir, self.seed, cx, cy);
                 if path.exists() {