Walker reached a union value but none of the candidate branches matched the runtime type. branches is the list of schema-declared alternatives; step carries the PathStep.UnionBranch the walker was attempting to resolve. Schema-driven; no JSON analogue.
Human-readable diagnostic. Kept separate from toString so the default enum representation remains useful for structural inspection / pattern-matching-in-tests.
Human-readable diagnostic. Kept separate from toString so the default enum representation remains useful for structural inspection / pattern-matching-in-tests.