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).
A Rule was exactly a Check<Model.Value, Model.Value>: it fired on every
node, switched on the value itself, and carried the root. It was therefore a
second copy of the one check machinery, the duplication the validation idiom
exists to prevent (principle 10), now also resolved for the tagged layer.
Remove Rule and its Context. The model validator holds only checks; its
default is the type-directed BuiltinCheck.mappingKeysAreUnique; validating
takes a Check. Custom whole-tree rules become Check<Model.Value, Model.Value>
with the former context.root now context.document.
Behaviour is byte-identical: the mapping-key builtin reports the same issues
at the same paths in the same DFS order, so the public validate/collect/blank
surface is unchanged. All 399 tests pass; the coverage gate reconciles to 60
public types.
