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.
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