despawn method
Despawns an entity, making its ID available for reuse.
Returns true if the entity was alive, false if already despawned or the generation doesn't match.
Implementation
bool despawn(Entity entity) {
if (!isAlive(entity)) return false;
final meta = _meta[entity.id];
meta.location = null;
meta.generation++;
_freeList.add(entity.id);
_aliveCount--;
return true;
}