A complexContent extension that adds element content (a non-empty model
group) produces an element-only/mixed content type, so its base must have
complex or empty content, not simpleContent (cos-ct-extends.1.4.2.2). Such
schemas were accepted.
The check requires the extension to add actual element content (an element or
wildcard, not an empty group), so an attribute-only complexContent extension
of a simpleContent base stays valid (the complexContent-around-simpleContent
idiom, e.g. particlesZ031).
XSTS invalid-schemas-accepted 235 to 233, no other bucket moved.
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.
