The tagged-node validator carried its own copy of the validation machinery:
a Tagged.Rule cloning Validation's check type and a Tagged.Context cloning
its context, differing only in subject type. That is the per-layer
duplication the validation idiom exists to prevent (principle 10).
Generalise the type-directed framework over both the subject and the
document: SubjectContext, Check, and AnyCheck now take a Document type
parameter for the validated tree's root. The model layer validates with a
Model.Value document, the tagged layer with a Tagged.Node document, each
truthful rather than forcing one root type to stand in for the other.
The tagged layer keeps only what is genuinely its own: the Node document
type, its built-in check (now Tagged.BuiltinCheck, a Check<Node, Node>), and
its traversal. Tagged.Rule and the tagged Context are removed. The validator
surface is unchanged but validating now takes a Check<Node, Node>.
All 399 tests pass; the coverage gate reconciles to 62 public types.
Mihaela M
fc1a6db1dc