Conform cultcache-rs to the store-composition contract, following C#:
- add_backing_store returns Err, attaching nothing, when the new store
would move the home of any entry type the cache already holds.
- pull_all_backing_stores loads every store into a staged image and
replaces the cache's entries only when all loaded. An entry delivered by
a store that is not its home returns Err naming the key and both
stores; nothing is admitted and no store is written.
- A cache with zero stores is in memory: put, put_envelope,
put_raw_envelope, put_prepared_batch and delete no longer error. Once
any store is attached, a type with no home still errors.
Tests for each rule fail on 6bb41f3.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
03ff299233