When a mob dies in combat, create_corpse is called to spawn a corpse
at the mob's position with the mob's inventory transferred. This
replaces the direct despawn_mob call, making combat deaths leave
lootable corpses behind.
The fallback to despawn_mob is kept if the mob somehow has no zone.
Corpse is a non-portable Container subclass that holds a deceased mob's
inventory. The create_corpse factory transfers items from the mob to the
corpse, sets a decompose_at timestamp for eventual cleanup, and calls
despawn_mob to remove the mob from the world.
