Introduce the core of the validation-rules idiom: a Check<Subject> with a
positive description, a when predicate, and a Bool or multi-error check
that fires only on values of its Subject type wherever they occur. AnyCheck
erases the subject and dispatches by runtime type, guarding an optional or
a wrong type from matching. The validator gains a checks list, a
validating(_:) overload, and a traversal that offers each node the values
it may specialise on. BuiltinCheck.mappingKeysAreUnique is the
type-directed counterpart of the legacy rule.
The machinery has its own negative tests (wrong type, optional, false
predicate yield nothing; same type twice yields two errors) and the builtin
ships a failing and a near-miss test. The coverage gate catches the five
new public types, which are classified as validation machinery. The default
validator is unchanged, so this is additive; retiring the legacy Rule,
moving the default onto the check, the combinator algebra, the error
rendering, and de-duplicating the Tagged validator are the remaining steps.
Mihaela M
2e45ecaffe