isUndeclaredReferenceRef fell through to a lenient accept when a ref or
substitutionGroup resolved to the XSD namespace but the schema did not target
it. The XSD namespace declares no referenceable user component (element,
group, attribute, attribute group), so such a reference is undeclared, the
asymmetry isUndeclaredReferenceType already handled for type/base.
This is reached when an unprefixed reference picks up a default xmlns of the
XSD namespace, so it is not in the target namespace where the schema's own
components live: xmlns="http://www.w3.org/2001/XMLSchema" targetNamespace="foo"
with <element ref="root"/> resolves to {XSD}root, not {foo}root (XSTS
xsd013/xsd014). A schema that itself targets the XSD namespace is the
inTargetNamespace case, and a genuinely loaded XSD-namespace document is the
foreignPools case before this branch, so both are unaffected.
Verified against the full W3C XSTS 2006-11-06 suite: invalid-schemas-accepted
29 -> 27, valid-schemas-rejected held at 0, no other bucket moved. Adds
reference tests for both reject cases and a qualified-ref guard.
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.
