CacheBackingStore.HeaderFor(holdsIds, existingHeader, wholeStore, directoryStore) replaces FormatFor,
DirectoryFormatFor and the writers' private marker logic. A writer that rewrites the whole store
decides by content; a writer that touches part of it keeps a header already marked. Flush,
unconditional and conditional commit, the directory store and CultMesh's single-file write all call
it. A typed CultMesh write replaces the file with a document it can see, so it decides by content;
a raw payload is opaque and keeps a marked file marked.
A record's HoldsIds is what its stored form persists: a load keeps a variant's delta as read, so
the ids a load mints in an override exist only in memory and no longer mark the store. A
conditional commit onto the file no longer reads this cache's view of what the file held, so a
stale view cannot over-mark it. IdPlan.HoldsIds is derived from HeldIds and the fills.
Reverts InternalsVisibleTo GameCult.Caching.MessagePack and the public DirectoryFormatV4/V5
constants: the directory store reaches the decision through the protected member it inherits.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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