Ship the OpenAPIKit-style validation layer (builtin rules, warnings channel,
Validator tiers, coverage gates) and wire XSD compile checks through it. Advance
#161 with chameleon include validation, multi-container reference resolution,
and include targetNamespace composition rules so merged schema sets compile
correctly when a schemaLoader is provided.
An element declaring substitutionGroup="head" whose own type is unrelated to
the head's type was accepted; XSD 1.0 e-props-correct.4 requires the member's
type be validly derived from the head's. The check runs after named types
resolve and reuses the restriction/extension + built-in-lattice derivation
oracle (an ur-type head admits any member).
It is conservative so it never rejects a valid member: it runs only for a
self-contained schema (no import/include/redefine, where the derivation table
is complete and single-namespace names are unambiguous), reads the member's
type from its own type attribute and the head's from the top-level element
declarations only (so a local element of the same name supplies neither), and
stands down when either type is a list or union (cos-st-derived-ok clauses
2.3/2.4 union-member derivation is not modelled).
The post-namedTypes consistency calls move into a postNamedTypeErrors
aggregator alongside consistencyErrors. XSTS: invalid schemas accepted
480 to 479, no other bucket raised.
