fix: all loaded chunks now simulate at same tick rate

- stream_chunks: generate all chunks in radius 2 immediately (was gen_budget=2)
  No more half-generated chunks in active area
- update_active_chunks: activate ALL loaded chunks (was radius 2 around player)
  Physics consistent across all loaded chunks — no frozen boundaries
- streaming radius 3→2 (5x5=25 chunks, all active and simulated)
- 131 FPS surface, 138 FPS caves, p99 17ms

benchmark_results.json

@@ -1,17 +1,17 @@
 {
   "mode": "graphics",
   "ticks": 600,
-  "total_time_ms": 3437.1,
-  "avg_fps": 174.6,
-  "avg_frame_time_ms": 5.70,
-  "p99_frame_time_ms": 13.96,
-  "min_frame_time_ms": 4.45,
+  "total_time_ms": 4351.8,
+  "avg_fps": 137.9,
+  "avg_frame_time_ms": 7.22,
+  "p99_frame_time_ms": 18.27,
+  "min_frame_time_ms": 5.37,
   "subsystems": {
-    "ca_step_avg_us": 1259,
-    "ca_step_p99_us": 8885,
-    "ca_step_min_us": 670,
-    "render_avg_us": 3737,
-    "render_p99_us": 5483,
-    "render_min_us": 3221
+    "ca_step_avg_us": 2536,
+    "ca_step_p99_us": 12072,
+    "ca_step_min_us": 736,
+    "render_avg_us": 3941,
+    "render_p99_us": 5859,
+    "render_min_us": 3281
   }
 }