`artwork explain` rejected `dc` and `mf` because it validated against RecheckKinds,
the list of kinds the backfill revisits. Those are different questions: a kind with no
recheck path still has artwork someone can report as wrong.
Disc artwork now walks DiscArtPriority under a trace, so explain reports which entry won
and why the others lost, including entries that map to no source at all (external is
unsupported, a disc with no subtitle, an album folder with no images). Media file artwork
traces its single embedded candidate, separating "EnableMediaFileCoverArt is off" from
"the track has no embedded art" — stored state cannot tell those apart.
Each command now validates against the kinds it can actually serve: explain takes all six,
refresh takes artwork.RefreshableKinds (which nativeapi now shares instead of keeping its
own copy), reprocess still takes RecheckKinds. Disc artwork stays out of refresh: the
worker cannot resolve it, so the queue row would be rejected on every drain.
WalksPriorityChain becomes Explainable, and ResolveArtist/ResolveAlbum collapse into
Resolve(kind, id).
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