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.
A configured 'external' token vanished from the chain when no enabled agent provided
images for that entity type, and for synthetic artists, leaving the trace unable to
say whether the tier was even considered.
