Walker resolved a union branch to an enum value whose runtime symbol is not a member of the schema's declared symbol set (EnumSymbol doesn't validate at construction, so hand-built payloads can carry one). 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.