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.

engine/collision_system.py

@@ -9,6 +9,19 @@ This structure is designed to be easily portable to Nim.
 from typing import Dict, List, Tuple, Set
 from dataclasses import dataclass
 
+
+def shrink_bbox(bbox: Tuple[float, float, float, float], ratio: float) -> Tuple[float, float, float, float]:
+    """Return a smaller bbox centered on the original one.
+
+    ratio is the fraction of width/height removed from each side.
+    E.g. ratio=0.25 keeps the central 50% of the original area.
+    """
+    x1, y1, x2, y2 = bbox
+    dx = (x2 - x1) * ratio / 2
+    dy = (y2 - y1) * ratio / 2
+    return (x1 + dx, y1 + dy, x2 - dx, y2 - dy)
+
+
 @dataclass
 class CollisionLayer:
     """Define which types of units can collide with each other."""