SingleFileBackingStore gains the cacheless single-file read and write CultMesh used through internals; CultMeshSingleFileDocuments is now a caller of the public API only, and the GameCult.Mesh InternalsVisibleTo grant (with the duplicate Tests/MessagePack grants) is deleted. Owns finding mesh-ivt-moved-not-removed.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
