The validation module already implemented the composable-rule idiom under
its own names (Check/Issue/SubjectContext) and a value-type Validator with
duplicated traversal machinery per layer. Renamed the core types to the
canonical names (Validation/ValidationError/ValidationErrorCollection/
ValidationContext/BuiltinValidation/AnyValidation), converted Validator to
a reference-type class with the two-tier default-rule split, and genericized
it over both Subject and Document via a new Traversable protocol so the
model and tagged layers share one walk engine instead of two copies. Wired
the JSON-representability rule set into the same shape and closed the two
missing pieces (unwrap combinator, JSON coverage/config-pin tests).
