Vendoring
mkContract { required; optional; description; } builds a typed
VendoringContract declaring the structural requirements a
vendored package must satisfy. defaultContract is the framework's
standard contract (name, deps required; version, path,
source/registry markers optional).
checkRequired contract metadata returns the list of contract-
required fields missing from metadata (empty list ⇒ valid).
define { langName; contract; … } produces a typed
VendoringBuilder spec for downstream composition.