Completes Stage 2's populate path. Still dormant — no consumer calls it yet.
- resolve_and_store_canonical_for_album(db, album_id, ...): loads the album's
source IDs + its tracks' (duration_ms, title) from the DB via the SAME
loader the Reorganizer uses (load_album_and_tracks + _extract_source_ids), so
the canonical is chosen over exactly the source IDs the reorganizer sees;
scores off the DB track rows (the library's view of the files — no per-file
disk reads), resolves the best fit, and persists it. Returns the stored result
or None when unresolved.
- default_fetch_tracklist(): production fetcher wrapping
get_album_tracks_for_source, normalising to {title, track_number, duration_ms}
(duration best-effort; sec->ms; absent -> scorer leans on count+title).
Design note: chose LAZY resolution (Stages 3-4 consumers call this when they hit
an album with no canonical) over a standalone backfill repair job — no new
scheduling/UI surface, resolves only when a tool actually needs it, and stays
gated (NULL canonical = today's behavior).
Tests: tests/test_canonical_orchestration.py (5) — end-to-end on a real temp DB
(11 files pick the 11-track release over a 17-track deluxe and persist it),
no-source-ids -> None, missing-album -> None, and default_fetch_tracklist
normalization (dict items, seconds->ms) + failure -> None. All canonical +
DB-migration tests green.
