cacheKey property
Storage key the cache is written/read under. Required whenever
storage is provided — see the constructor's ArgumentError. Not a
named StorageKeys constant (unlike most storage keys in this repo)
because a caller may construct several packs (different content
types/asset paths) that each need their own cache slot; the caller
picks a unique key the same way SaveSlotManager/CheckpointCoordinator
let callers supply their own scoped key.
Implementation
final String? cacheKey;