feat: explosions, particle system, electricity, structural integrity

Explosions:
- trigger_explosion(x, y, radius, damage) in Game
- Destroys non-static solids, ignites empty cells, pressure spike
- Knockback to entities (velocity away from center + upward pop)
- Particle burst on explosion (fire + smoke + debris)

Particle system:
- ParticleManager in src/physics/particle.rs (capacity 2000)
- CPU-side: position, velocity, life, color, gravity, size
- spawn_burst() for radial bursts, individual spawn() for ambient
- Rendered as third draw call in graphics.rs (is_ui: 2 in shader)
- ParticleInstance = ColorInstance compatible (12 bytes)
- upload_particles() via GpuRenderer trait
- Ambient: fire sparks, lava bubbles — scanned around player

Electricity:
- chunk.electricity: Vec<u8> (4KB/chunk) — current strength 0-255
- Material.conductive + conductivity fields (water = conductive)
- electricity_step() in CA: propagates current through conductive
  neighbors, decays over time, skips non-conductive cells
- cells_swap swaps electricity, serialization saves/loads it

Structural integrity:
- Material.structural: bool (stone, wood = true)
- structural_step() in CA (every 30 ticks, infinite mode only):
  checks if structural cells have support (below, below-left, below-right)
  converts unsupported to Sand (falls)
- Only active in infinite mode to avoid breaking test grids

Graphics shader: is_ui==2 branch renders particles in world-space
All 185 tests + 14 scenarios pass. 143 FPS benchmark.

benchmark_results.json

@@ -1,17 +1,17 @@
 {
   "mode": "graphics",
   "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,
+  "total_time_ms": 2104.1,
+  "avg_fps": 142.6,
+  "avg_frame_time_ms": 6.99,
+  "p99_frame_time_ms": 15.98,
+  "min_frame_time_ms": 5.43,
   "subsystems": {
-    "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
+    "ca_step_avg_us": 2900,
+    "ca_step_p99_us": 11976,
+    "ca_step_min_us": 1280,
+    "render_avg_us": 3459,
+    "render_p99_us": 5031,
+    "render_min_us": 3155
   }
 }