Summary

Fixes persistent entity count drift where limits would show more mobs
than actually existed on the island. After a recount (/calc) the real
(lower) count would appear.

Changes

entityIslandMap + EntityAddToWorldEvent

Maps entity UUID to island ID so decrement always targets the correct
island, even when the entity wanders off-island before dying. The map
was transient — lost on every restart. An EntityAddToWorldEvent handler
now repopulates it when chunks load. A justPortaled debounce list
prevents double-decrement if EntityRemoveEvent fires for the
source-world removal during a portal teleport.

Entity change persistence

incrementEntity/decrementEntity called setChanged() but never triggered
a save — only block changes called updateSaveMap. Entity-only changes
were only persisted on onDisable(). Added markChanged() on
BlockLimitsListener so entity changes join the batch-save cycle
(every 10 changes), ensuring counts survive restarts even without
block activity on the island.

Testing

132 tests pass across EntityLimitListenerTest, BlockLimitsListenerTest,
JoinListenerTest, PaperShulkerLimitListenerTest, and others.