Atomic presence-aware lookup: resolves to a CacheHitEnvelope when
the key is cached (even for a stored undefined) or to undefined on a
miss. This is the cache middleware's read primitive — it answers presence
and value in a single backend round-trip, so it must be implemented by
every provider (no racy get()+has() probe).
OptionalhasInvalidates concrete storage keys after the caller has already normalized and identity-scoped them.
Invalidates semantic refs after the caller has already normalized them.
Optionalmetadata: CacheEntryMetadata
Low-level cache instance contract used by task cache middleware.