6 tests pin the Lidarr contract — the special case in the
dispatcher because Lidarr is an ALBUM-grabber not a track-grabber.
Filename format is `album_foreign_id||display` (MusicBrainz album
MBID Lidarr uses for lookups). State dict is SMALLER than streaming
sources (no track_id, no transferred/speed — Lidarr polls its own
queue API for byte-level progress). Thread target signature is
3-arg, no original_filename. Engine refactor's plugin contract
must accommodate album-only sources or Lidarr stays special.
