Closes the kettui gap — the orchestration was unproven. Injected-fake seam
tests (temp sqlite + real empty track files, no metadata APIs / no real tag
writes):
- embed_known_source_ids: builds the right canonical id_tags from flat db keys,
honors the musicbrainz embed gate, no-ops when there's nothing to write.
- library_retag scan: produces a detailed finding with the per-track old->new
diff + stamped source ids, and skips an album that's already correct.
- _add_source_ids: per-source key mapping.
- _fix_library_retag apply: writes each track's payload, and reports failure
when files are unreachable.
476 tests pass; ruff clean.
