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/mine.py

@@ -3,6 +3,8 @@ from .bomb import Explosion
 from engine.collision_system import CollisionLayer
 
 class Mine(Unit):
+    draw_on_top = True
+
     def __init__(self, game, position=(0,0), id=None):
         super().__init__(game, position, id, collision_layer=CollisionLayer.MINE, unit_type=UnitType.MINE)
         self.speed = 1.0  # Mine doesn't move but needs speed for consistency