Store gate: Cache owns the attach window; pin the three-gate bound; fix the unseen-cache wording

A cacheless write that re-enters its own store's hold now consults Cache first, so a journal's WriteDocument or WriteRecord gets the same refusal during an attach as after it (the attached text); the re-entry text no longer names an attach's pull. InStoreHold takes the store instead of its gate.
CacheBackingStore.GateTakes (internal, thread-static) counts the gates Gate takes; OneStoreCallTakesAtMostThreeGatesUnderFailingAttachChurn asserts at most three per call under failing-attach churn, which pins the stale-guess loop (M4b).
The contract and AddBackingStore say what holds: other threads may guess the tentative cache, but none runs a body against it.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

unity/org.gamecult.cultlib/CHANGELOG.md

@@ -23,7 +23,7 @@ Releases as 3.0.0 (major).
 
 ### Added
 
-- A cacheless `WriteRecord` or `WriteDocument`, and an attach, hold the store's gate while they run, and user code they run (a document's property getters, a constructor while a copied record is read) now throws `InvalidOperationException` if it enters a cache it does not hold or a second store; it used to wait forever against an attach of the same store. A store call that queued on an attach that then failed runs under the gate that owns the store, not beside a cacheless call or a second cache's attach. An attach that fails takes its registry back, so a `PullAll` on the store afterwards is refused as unattached instead of loading records the next cache never sees. A cacheless `WriteRecord` or `WriteDocument` made while the same thread already holds that store (from a journal an attach's pull runs, or from user code another cacheless write of the store runs) throws `InvalidOperationException`; it used to write the file behind the attaching cache and take that cache's registry, so the cache could never flush or pull the store again. A journal that calls its cache's store (`Push`, `Delete`, `PushAll`, `CommitBatch`, `PullAll`, `ReadStoreSnapshot`, `JudgeChecks`) while that cache's `AddBackingStore` runs the journal now throws "A journal entered the cache it journals", as it does after the attach; it used to stage or write records the cache never admitted, so the cache and the file disagreed.
+- A cacheless `WriteRecord` or `WriteDocument`, and an attach, hold the store's gate while they run, and user code they run (a document's property getters, a constructor while a copied record is read) now throws `InvalidOperationException` if it enters a cache it does not hold or a second store; it used to wait forever against an attach of the same store. A store call that queued on an attach that then failed runs under the gate that owns the store, not beside a cacheless call or a second cache's attach. An attach that fails takes its registry back, so a `PullAll` on the store afterwards is refused as unattached instead of loading records the next cache never sees. A cacheless `WriteRecord` or `WriteDocument` made while the same thread already holds that store from user code another cacheless write of the store runs, or from a journal an attach's pull runs (then "The store is attached to a cache, write through the cache.", as after the attach) throws `InvalidOperationException`; it used to write the file behind the attaching cache and take that cache's registry, so the cache could never flush or pull the store again. A journal that calls its cache's store (`Push`, `Delete`, `PushAll`, `CommitBatch`, `PullAll`, `ReadStoreSnapshot`, `JudgeChecks`) while that cache's `AddBackingStore` runs the journal now throws "A journal entered the cache it journals", as it does after the attach; it used to stage or write records the cache never admitted, so the cache and the file disagreed.
 - `SingleFileBackingStore.ReadStoreSnapshot()` reads a store file once, under the store's in-process gate, with the reader a cache's store opens with, and returns the durable snapshot or null when nothing is at the path. `SingleFileBackingStore.WriteRecord(record, catalogEntry, holdsIds, registry)` writes one record without a cache: under the store's lock every other record and catalog entry is copied as stored, the file is replaced atomically, and the header is decided as a flush decides it. It refuses on a store attached to a cache.
 - `SingleFileBackingStore.WriteDocument<T>(key, document, storedAt, registry)` writes one typed document without a cache, as a cache write does: it plans element ids, encodes the document as `T` (the type its schema id and ids are taken from, also for a derived instance) and marks the store when it holds an id; a failed write gives the minted ids back. It refuses on a store attached to a cache. The `registry` of `WriteRecord` and `WriteDocument` serves that call only; a store keeps none.
 - `CultCodec.DecodeDocument<T>(key, payload)` decodes a stored payload as a cache load does (`CultCacheMessagePack.CreateCodec(registry)` makes the codec). It reads no file and holds no state. `CultElementIds` stays internal.