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).
Routes the value and stream APIs through the ported scanner+parser (the
event API already was), so PureYAML now parses entirely on the libyaml
port. Adds the features needed to match and exceed the previous engine:
JSON UTF-16 surrogate-pair \u escapes, bare documents after an explicit
`...`, reserved-directive handling that still requires a following `---`,
and a 64-level nesting bound enforced on the event stream so deeply nested
input fails with nestingTooDeep instead of overflowing a recursive
consumer. Parser errors are translated to the diagnostic-aware ParseError,
recovering precise codes where identifiable and a readable malformed
fallback otherwise; error-reporting fixtures are rebased onto the port.
Canonical event conformance: 268 -> 271 of 308 (7Z25, HWV9, QT73).
393 tests pass.
