Fix gas poisoning, explosion hit detection and render order

- Move gas poisoning from Gas.move() to Gas.collisions() so all rats are
  registered in the collision system before the poison query runs.
- Shrink gas and explosion bboxes so rats must be well inside the tile to
  be poisoned/killed.
- Use AABB overlap instead of partial_move threshold for gas poisoning.
- Draw mobile units first, then top-layer effects (gas, mines, bombs,
  explosions) so rats appear under the gas.
- Add draw_on_top hint to Unit base class and top-layer units.

units/gas.py

@@ -1,11 +1,14 @@
 from .unit import Unit, UnitType
-from engine.collision_system import CollisionLayer
+from engine.collision_system import CollisionLayer, shrink_bbox
 import random
 
 # Costanti
 AGE_THRESHOLD = 200
+GAS_BBOX_SHRINK = 0.3  # central 40% of the tile is poisonous
 
 class Gas(Unit):
+    draw_on_top = True
+
     def __init__(self, game, position=(0,0), id=None, parent_id=None):
         super().__init__(game, position, id, collision_layer=CollisionLayer.GAS, unit_type=UnitType.GAS)
         self.parent_id = parent_id
@@ -25,27 +28,14 @@ class Gas(Unit):
             return
         self.age += 1
         
-        # Use optimized collision system to find rats in gas cloud
-        victim_ids = self.game.collision_system.get_units_in_cell(
-            self.position, use_before=False
+        # Register a smaller bbox so rats must be well inside the tile to be gassed
+        cell_bbox = (
+            self.position[0] * self.game.cell_size,
+            self.position[1] * self.game.cell_size,
+            (self.position[0] + 1) * self.game.cell_size,
+            (self.position[1] + 1) * self.game.cell_size,
         )
-        
-        for victim_id in victim_ids:
-            victim = self.game.unit_manager.get_unit_by_id(victim_id)
-            if victim and victim.type in [UnitType.RAT_MALE, UnitType.RAT_FEMALE]:
-                if victim.partial_move > 0.5:
-                    victim.gassed += 1
-        
-        # Check position_before as well
-        victim_ids_before = self.game.collision_system.get_units_in_cell(
-            self.position, use_before=True
-        )
-        
-        for victim_id in victim_ids_before:
-            victim = self.game.unit_manager.get_unit_by_id(victim_id)
-            if victim and victim.type in [UnitType.RAT_MALE, UnitType.RAT_FEMALE]:
-                if victim.partial_move < 0.5:
-                    victim.gassed += 1
+        self.bbox = shrink_bbox(cell_bbox, GAS_BBOX_SHRINK)
         
         if self.age % self.speed:
             return
@@ -61,8 +51,15 @@ class Gas(Unit):
                     if not any(unit.type == UnitType.GAS for unit in self.game.units.values() if unit.position == (new_x, new_y)):
                         print(f"Spreading gas from {self.position} to ({new_x}, {new_y})")
                         self.game.unit_manager.spawn_unit(Gas, (new_x, new_y), parent_id=self.parent_id if self.parent_id else self.id)
+
     def collisions(self):
-        pass
+        """Poison rats that overlap the shrunk gas tile in Pass 2."""
+        for _, victim_id in self.game.collision_system.get_collisions_for_unit(
+            self.id, CollisionLayer.GAS, tolerance=0
+        ):
+            victim = self.game.unit_manager.get_unit_by_id(victim_id)
+            if victim and victim.type in [UnitType.RAT_MALE, UnitType.RAT_FEMALE] and victim.id in self.game.units:
+                victim.gassed += 1
 
     def die(self, unit=None, score=None):
         if not unit: