detect_album_info_web gains a last-resort step: when a track matched a SINGLE
with no usable album context, look up the parent ALBUM that contains it (via
get_artist_albums_for_source + get_artist_album_tracks) and promote to it, so it
groups with its album-mates and gets the album's cover instead of the single's.
GATED behind metadata_enhancement.single_to_album (default OFF) — it's a
per-import metadata lookup, so it's opt-in, matching the canonical-version
pattern. Fully fail-safe: flag off, no source, or any client error/miss -> None,
so the track stays exactly as matched (never worse than today). The promoted
album name is forced past get_import_clean_album (which otherwise pins the
single's name) so grouping + tags use the album. 4 glue seam tests added
(promote-when-enabled, disabled-by-default, no-match, client-raises); 462
import-suite tests pass.
When a track matches a SINGLE release it carries the single's name/id and the
canonical grouping files it apart from its album-mates -> mixed cover art
(Sokhi). This re-homes it onto the album that actually contains it.
The selection is a pure, CONSERVATIVE function and the lookup loop takes injected
fetchers, so both are unit-testable without a live client. It only re-homes a
track when a real 'album'-type release's tracklist contains that EXACT track
(qualifier-tolerant) — never promotes a genuine standalone single, never guesses
(a wrong promotion would mis-home a real single, the inverse bug). Fail-safe: any
miss/error -> None (track stays as matched). 13 seam tests. Wiring next.
