v1.1: Performance optimizations and bug fixes

Major improvements:
- NumPy-based collision system supporting 200+ units (~3ms/frame)
- Spatial hashing with vectorized distance calculations
- 4-pass game loop ensuring correct collision timing
- Blood overlay system with pre-generated stain pool
- Cached render positions and viewport bounds
- Spawn protection preventing rats spawning on weapons

Bug fixes:
- Fixed bombs not killing rats (collision system timing)
- Fixed gas not affecting rats (collision system timing)
- Fixed rats spawning on weapons (added has_weapon_at check)
- Fixed AttributeError with Gas collisions (added isinstance check)
- Fixed blood stain transparency (RGBA + SDL_BLENDMODE_BLEND)
- Reduced point lifetime from 200 to 90 frames (~1.5s)
- Blood layer now clears on game restart

Technical changes:
- Added engine/collision_system.py with CollisionLayer enum
- Updated all units to use collision layers
- Pre-allocate NumPy arrays with capacity management
- Hybrid collision approach (<10 simple, ≥10 vectorized)
- Python 3.13 compatibility

engine/unit_manager.py

@@ -4,7 +4,16 @@ from units import gas, rat, bomb, mine
 
 
 
-class UnitManager:        
+class UnitManager:
+    def has_weapon_at(self, position):
+        """Check if there's a weapon (bomb, gas, mine) at the given position"""
+        for unit in self.units.values():
+            if unit.position == position:
+                # Check if it's a weapon type (not a rat or points)
+                if isinstance(unit, (bomb.Timer, bomb.NuclearBomb, gas.Gas, mine.Mine)):
+                    return True
+        return False
+        
     def count_rats(self):
         count = 0
         for unit in self.units.values():
@@ -24,6 +33,19 @@ class UnitManager:
     def spawn_rat(self, position=None):
         if position is None:
             position = self.choose_start()
+        
+        # Don't spawn rats on top of weapons
+        if self.has_weapon_at(position):
+            # Try nearby positions
+            for dx, dy in [(0,1), (1,0), (0,-1), (-1,0), (1,1), (-1,-1), (1,-1), (-1,1)]:
+                alt_pos = (position[0] + dx, position[1] + dy)
+                if not self.map.is_wall(alt_pos[0], alt_pos[1]) and not self.has_weapon_at(alt_pos):
+                    position = alt_pos
+                    break
+            else:
+                # All nearby positions blocked, abort spawn
+                return
+        
         rat_class = rat.Male if random.random() < 0.5 else rat.Female
         self.spawn_unit(rat_class, position)