feat: camera zoom, day/night cycle, tooltip, crosshair

- Camera zoom: +/- keys adjust cell size 4-24px, grid_w/h recalculated
  Works in both graphics and ascii renderers via GpuRenderer trait
- Day/night cycle: ambient_light_at_tick() modulates brightness by sine
  Applied to terminal renderer (CPU lighting path)
- Tooltip: shows material name + temperature at mouse cursor position
  Tracked via CursorMoved event, world coords computed from screen pos
- Crosshair: dotted line from player to mouse cursor, fading alpha
  Shows aiming trajectory for projectile combat

All 185 tests + 14 scenarios pass. 162 FPS benchmark.

benchmark_results.json

@@ -1,17 +1,17 @@
 {
   "mode": "graphics",
-  "ticks": 600,
-  "total_time_ms": 4229.7,
-  "avg_fps": 141.9,
-  "avg_frame_time_ms": 7.02,
-  "p99_frame_time_ms": 17.35,
-  "min_frame_time_ms": 5.45,
+  "ticks": 300,
+  "total_time_ms": 1847.9,
+  "avg_fps": 162.3,
+  "avg_frame_time_ms": 6.13,
+  "p99_frame_time_ms": 15.95,
+  "min_frame_time_ms": 4.61,
   "subsystems": {
-    "ca_step_avg_us": 2425,
-    "ca_step_p99_us": 11400,
-    "ca_step_min_us": 600,
-    "render_avg_us": 3863,
-    "render_p99_us": 5805,
-    "render_min_us": 3218
+    "ca_step_avg_us": 1708,
+    "ca_step_p99_us": 11597,
+    "ca_step_min_us": 332,
+    "render_avg_us": 3717,
+    "render_p99_us": 6538,
+    "render_min_us": 3295
   }
 }