ed4178a6 gated serveDisc on len(album.Discs) > 1, claiming parity with the legacy
reader. The legacy reader has no such gate: artwork.go dispatches every dc- id to
newDiscArtworkReader, whose Reader() walks DiscArtPriority unconditionally.
The gate also lost art. For a single-disc album whose only image is disc1.jpg,
the disc request skipped the chain and fell through to album art, which does not
match CoverArtPriority — so tracks tagged disc 1 (whose CoverArtID is a dc- id)
served nothing at all, where before they served disc1.jpg.
A single disc can legitimately have its own cover, distinct from the album's, and
DiscArtPriority is what expresses that preference. Drop the gate and restore the
single-disc e2e scenarios that covered it.
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