Class LocalArtifactLoader

The single production IArtifactLoader — the read-side inverse of MJFilesArtifactStore. Given a fileId (= the real MJ: Files row id the artifact was stored under), it reads the bytes from local disk at <baseDir>/<fileId>.bin and returns them, or null when the file is absent.

A missing file is a normal, expected null (the model has no persisted artifact); only a genuine I/O failure (permissions, corruption) is logged.

NOT for multi-host production — it can only read paths on the machine the artifact was written to. Production follow-up: replace the local readFile with a provider GetObject keyed by the File id; the id contract is unchanged.