- introduce the in-process metadata engine and fresh provider adapters
- normalize external identity through source_id while keeping id as a compatibility alias
- route metadata callers through the new seam and add regression coverage
Three more album-shape consumers now route through
Album.from_<source>_dict() when caller passes a known source:
- _build_discography_release_dict (artist discography cards)
- _build_artist_detail_release_card (artist detail release cards)
- _normalize_track_album (quality scanner result normalization)
Legacy duck-typing stays as fallback for unknown source,
non-dict input, or converter errors. Pure additive — existing
callers without source kwarg unchanged.
- split metadata lookup logic into core/metadata/*
- keep core/metadata_service.py as the legacy barrel
- update tests and artist-detail code to patch concrete modules
