Only cache artwork results that were matched by album. Previously the
first track looked up in an album decided the cached answer for every
other track: the title-only fallback (and its miss) was stored under the
album key, so later tracks in the album inherited it even when a
title-specific lookup would have found artwork.
fetchAnimatedArtwork now returns a scope key alongside the result — the
album key only when album-matched, else the title key. The dedupe skips
in artwork.ts and index.ts compare against that effective scope, so
skipping within an album stays free whenever the album matched.