Raid handles

Raid handles returned by LevelRef::raid, raid_at and raids resolve their own loaded server level even while another dimension is in place. Raid reads, omen-level writes and stop do not swap levels. Raider lists and living health come from that level's mobs; raid ids are local to a level. A closed level, an earlier world epoch or a client refuses the handle. Entity handles in a parked raid's member list keep their identity; their ordinary entity operations still require the entity's level to be in place.