Removes FlushBackingStore, both PrepareForReloadOrShutdown overloads,
PullBackingStoreRecordsAsync, TryGet<T>, TryGetByName, TryGetByIndex,
Resolve<T>, the cache and store Logger, and LastSuccessfulFlushAtUtc on the
cache and stores. GetStoredDocuments<T> folds into GetAll<T>.
The soft flag goes from FlushAllBackingStores, ExecuteTransactionAsync,
CommitBatch and PushAll. It stays on FlushAsync(bool) because
CultCacheStudioWindow reflects on that signature until Cut 6.
CultMesh.FlushAsync and CultNetLocal.FlushAsync keep their parameter and
stop forwarding it.
The store's EntryAdded/EntryUpdated/EntryDeleted subjects become
protected internal Loaded/Unloaded callbacks set by the cache at attach.
AddStoredDocumentInternal is synchronous, so the GetAwaiter().GetResult()
bridges on the store subscriptions and the global materializer are gone.
Tests: drop the LastSuccessfulFlushAtUtc asserts; OpenAsync and GetAll tests
read through Get/GetAll; the persisted-global test proves the durable global
loads without staging a default instead of going through selective hydration;
the lease-before-observers test observes through CultCache.Watch; the
PullSelected selective-hydration test is deleted.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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