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

units/rat.py

@@ -19,6 +19,7 @@ class Rat(Unit):
         self.speed = 0.10  # Rats are slower
         self.fight = False
         self.gassed = 0
+        self.direction = "DOWN"  # Default direction
         # Initialize position using pathfinding
         self.position = self.find_next_position()
 
@@ -72,6 +73,31 @@ class Rat(Unit):
             self.position = self.find_next_position()
         self.direction = self.calculate_rat_direction()
         
+        # Pre-calculate render position for draw() - optimization
+        self._update_render_position()
+    
+    def _update_render_position(self):
+        """Pre-calculate rendering position and bbox during move() to optimize draw()"""
+        sex = self.sex if self.age > AGE_THRESHOLD else "BABY"
+        
+        # Get cached image size instead of calling get_image_size()
+        image_size = self.game.rat_image_sizes[sex][self.direction]
+        
+        # Calculate partial movement offset
+        if self.direction in ["UP", "DOWN"]:
+            partial_x = 0
+            partial_y = self.partial_move * self.game.cell_size * (1 if self.direction == "DOWN" else -1)
+        else:
+            partial_x = self.partial_move * self.game.cell_size * (1 if self.direction == "RIGHT" else -1)
+            partial_y = 0
+        
+        # Calculate final render position
+        self.render_x = self.position_before[0] * self.game.cell_size + (self.game.cell_size - image_size[0]) // 2 + partial_x
+        self.render_y = self.position_before[1] * self.game.cell_size + (self.game.cell_size - image_size[1]) // 2 + partial_y
+        
+        # Update bbox for collision system
+        self.bbox = (self.render_x, self.render_y, self.render_x + image_size[0], self.render_y + image_size[1])
+        
     def collisions(self):
         """
         Optimized collision detection using the vectorized collision system.
@@ -94,6 +120,10 @@ class Rat(Unit):
         for _, other_id in collisions:
             other_unit = self.game.get_unit_by_id(other_id)
             
+            # Skip if not another Rat
+            if not isinstance(other_unit, Rat):
+                continue
+            
             if not other_unit or other_unit.age < AGE_THRESHOLD:
                 continue
             
@@ -128,25 +158,35 @@ class Rat(Unit):
         self.game.add_blood_stain(death_position)
 
     def draw(self):
-        start_perf = self.game.render_engine.get_perf_counter()
-        direction = self.calculate_rat_direction()
-        
+        """Optimized draw using pre-calculated positions from move()"""
         sex = self.sex if self.age > AGE_THRESHOLD else "BABY"
-        image = self.game.rat_assets_textures[sex][direction]
-        image_size = self.game.render_engine.get_image_size(image)
-        self.rat_image = image
-        partial_x, partial_y = 0, 0
+        image = self.game.rat_assets_textures[sex][self.direction]
         
-        if direction in ["UP", "DOWN"]:
-            partial_y = self.partial_move * self.game.cell_size * (1 if direction == "DOWN" else -1)
-        else:
-            partial_x = self.partial_move * self.game.cell_size * (1 if direction == "RIGHT" else -1)
+        # Calculate render position if not yet set (first frame)
+        if not hasattr(self, 'render_x'):
+            self._calculate_render_position()
         
-        x_pos = self.position_before[0] * self.game.cell_size + (self.game.cell_size - image_size[0]) // 2 + partial_x
-        y_pos = self.position_before[1] * self.game.cell_size + (self.game.cell_size - image_size[1]) // 2 + partial_y
-        self.game.render_engine.draw_image(x_pos, y_pos, image, anchor="nw", tag="unit")
-        self.bbox = (x_pos, y_pos, x_pos + image_size[0], y_pos + image_size[1])
+        # Use pre-calculated positions
+        self.game.render_engine.draw_image(self.render_x, self.render_y, image, anchor="nw", tag="unit")
+        # bbox already updated in _update_render_position()
         #self.game.render_engine.draw_rectangle(self.bbox[0], self.bbox[1], self.bbox[2] - self.bbox[0], self.bbox[3] - self.bbox[1], "unit")
+    
+    def _calculate_render_position(self):
+        """Calculate render position and bbox (used when render_x not yet set)"""
+        sex = self.sex if self.age > AGE_THRESHOLD else "BABY"
+        image_size = self.game.render_engine.get_image_size(
+            self.game.rat_assets_textures[sex][self.direction]
+        )
+        
+        partial_x, partial_y = 0, 0
+        if self.direction in ["UP", "DOWN"]:
+            partial_y = self.partial_move * self.game.cell_size * (1 if self.direction == "DOWN" else -1)
+        else:
+            partial_x = self.partial_move * self.game.cell_size * (1 if self.direction == "RIGHT" else -1)
+        
+        self.render_x = self.position_before[0] * self.game.cell_size + (self.game.cell_size - image_size[0]) // 2 + partial_x
+        self.render_y = self.position_before[1] * self.game.cell_size + (self.game.cell_size - image_size[1]) // 2 + partial_y
+        self.bbox = (self.render_x, self.render_y, self.render_x + image_size[0], self.render_y + image_size[1])
         
 class Male(Rat):
     def __init__(self, game, position=(0,0), id=None):