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)

benchmark_results.json

@@ -1,17 +1,17 @@
 {
   "mode": "graphics",
   "ticks": 600,
-  "total_time_ms": 4648.7,
-  "avg_fps": 129.1,
-  "avg_frame_time_ms": 7.72,
-  "p99_frame_time_ms": 11.03,
-  "min_frame_time_ms": 7.13,
+  "total_time_ms": 3483.9,
+  "avg_fps": 172.2,
+  "avg_frame_time_ms": 5.78,
+  "p99_frame_time_ms": 14.20,
+  "min_frame_time_ms": 4.20,
   "subsystems": {
-    "ca_step_avg_us": 1121,
-    "ca_step_p99_us": 1713,
-    "ca_step_min_us": 496,
-    "render_avg_us": 3543,
-    "render_p99_us": 4374,
-    "render_min_us": 3289
+    "ca_step_avg_us": 1397,
+    "ca_step_p99_us": 10172,
+    "ca_step_min_us": 303,
+    "render_avg_us": 3668,
+    "render_p99_us": 5514,
+    "render_min_us": 3259
   }
 }