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/bomb.py

@@ -1,16 +1,19 @@
 from .unit import Unit, UnitType
 from . import rat
 from .points import Point
-from engine.collision_system import CollisionLayer
+from engine.collision_system import CollisionLayer, shrink_bbox
 import uuid
 import random
 
 # Costanti
 AGE_THRESHOLD = 200
 NUCLEAR_TIMER = 50  # 1 second at ~50 FPS
+EXPLOSION_BBOX_SHRINK = 0.25  # central 50% of the tile is lethal instantly
 
 
 class Bomb(Unit):
+    draw_on_top = True
+
     def __init__(self, game, position=(0,0), id=None, unit_type=UnitType.BOMB_TIMER):
         super().__init__(game, position, id, collision_layer=CollisionLayer.BOMB, unit_type=unit_type)
         # Specific attributes for bombs
@@ -104,13 +107,24 @@ class Timer(Bomb):
             layer_filter=CollisionLayer.RAT
         )
 
+        cs = 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.id in self.game.units:
-                if victim.position in explosion_positions or victim.position_before in explosion_positions:
-                    victim.die(score=score)
-                    if score < 160:
-                        score *= 2
+                # Require the rat to be well inside an explosion tile
+                for pos in explosion_positions:
+                    if victim.position == pos or victim.position_before == pos:
+                        exp_bbox = shrink_bbox(
+                            (pos[0] * cs, pos[1] * cs, (pos[0] + 1) * cs, (pos[1] + 1) * cs),
+                            EXPLOSION_BBOX_SHRINK,
+                        )
+                        vb = victim.bbox
+                        if (vb[0] < exp_bbox[2] and vb[2] > exp_bbox[0] and
+                                vb[1] < exp_bbox[3] and vb[3] > exp_bbox[1]):
+                            victim.die(score=score)
+                            if score < 160:
+                                score *= 2
+                            break
 
     def draw(self):
         """Don't draw a bomb that has already exploded."""
@@ -130,10 +144,12 @@ class Explosion(Bomb):
         self.age += self.speed
         if self.age >= AGE_THRESHOLD:
             self.die()
-        # Set bbox so lingering explosions can kill rats via collision system
-        x = self.position[0] * self.game.cell_size
-        y = self.position[1] * self.game.cell_size
-        self.bbox = (float(x), float(y), float(x + self.game.cell_size), float(y + self.game.cell_size))
+        # Set bbox so lingering explosions can kill rats via collision system.
+        # Only the central area is lethal.
+        cs = self.game.cell_size
+        cell_bbox = (self.position[0] * cs, self.position[1] * cs,
+                     (self.position[0] + 1) * cs, (self.position[1] + 1) * cs)
+        self.bbox = shrink_bbox(cell_bbox, EXPLOSION_BBOX_SHRINK)
 
     def collisions(self):
         """Lingering explosion kills any rat that touches it."""