Fix bomb explosions not killing rats and remove tracked pycache

- Move Timer explosion from move() to collisions() so all units are
  registered in the collision system before the kill query runs.
- Explosion units now set a bbox and kill rats that touch them.
- Guard Rat.draw() so dead rats are not drawn.
- Remove units/__pycache__ files from tracking.

units/rat.py

@@ -171,6 +171,8 @@ class Rat(Unit):
 
     def draw(self):
         """Optimized draw using pre-calculated positions from move()"""
+        if self.id not in self.game.units:
+            return
         sex = self.sex if self.age > AGE_THRESHOLD else "BABY"
         image = self.game.graphics.rat_assets_textures[sex][self.direction]
         image_size = self.game.graphics.rat_image_sizes[sex][self.direction]