- 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.
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