`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).
Export RecheckKinds and add WalksPriorityChain so the CLI stops keeping its
own copies of both, and unexport externalCandidate, which nothing outside the
package consumes.
ChainTrace and TraceStep were unexported types re-exported through aliases,
which existed only so the CLI had a name to refer to them by. The types are
public API — Resolver.Steps returns []TraceStep and the CLI constructs a
ChainTrace — so name them that way and drop the indirection.
Encapsulation is unchanged: add, mu and steps stay unexported, so only this
package can write a step.
The CLI hardcoded the outcome literals and the "external:" prefix, so renaming a
constant's value in core/artwork left cmd compiling and the suite green while
`artwork explain` silently degraded its verdict.
Renaming a value now fails the golden vocabulary test in core/artwork and the
explainResult tests in cmd.
