A content-model violation is now placed at the offending child with a
recovery hint naming what was expected, instead of one opaque "content
does not match the content model" per element. The sequence/choice
automaton is walked child by child via its follow-set to flag the first
child the model rejects, or the incomplete content at the end; an xs:all
group recovers past each stray child and reports every missing required
member. Well-placed children are still validated for their own content,
so an editor sees every problem at once. Brings XSD to the same located,
recovering standard as the RELAX NG, DTD, and Schematron validators.