Class MJFilesArtifactStore

The single coherent IArtifactStore: it creates a real MJ: Files row (through the injected IEntityFactory, stamped with the active storage ProviderID) AND persists the serialized artifact bytes to local disk at <baseDir>/<file.ID>.bin. It returns the real File row id so MLModel.ArtifactFileID satisfies its FK to __mj.File, and the matching LocalArtifactLoader reads the bytes back by that id.

Why both halves: the FK forces a genuine File row id, but MJ: Files.Save() only records the metadata row — it never moves bytes — so for this dev / on-prem mode we write the bytes ourselves, keyed by the File id. Production follow-up: swap the local writeFile for a real provider PutObject; the id contract is unchanged.