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.

rats.py

@@ -516,7 +516,21 @@ class MiceMaze:
         # Pass 2: RESOLVE and DRAW
         for unit in sorted_units:
             unit.collisions()
-            unit.draw()
+
+        # Draw mobile units first so ground effects appear on top
+        for unit in sorted_units:
+            if not getattr(unit, "draw_on_top", False):
+                unit.draw()
+
+        # Draw top-layer effects on top of mobile units
+        for unit in sorted_units:
+            if getattr(unit, "draw_on_top", False) and not getattr(unit, "draw_last", False):
+                unit.draw()
+
+        # Draw foreground/last-layer units (points) on top of everything
+        for unit in sorted_units:
+            if getattr(unit, "draw_last", False):
+                unit.draw()
         
         self.graphics.draw_cave_foreground()
         self.render_engine.draw_pointer(self.pointer[0] * self.cell_size, self.pointer[1] * self.cell_size)