Content no content-model field matches is reported, not silently dropped

Found by WORK-625, the composed character. A {% hint %} an author wrote before the first section was matched by no preamble field and dropped, with no error and no warning. The composed definition works around it with a catch-all body preamble field. SPEC-145 records the finding beside the character worked example.

D11 promises against this silent loss but checks the other direction. It requires that every declared field is placed by a slot. Nothing checks that every authored node is matched by a field. So content can still vanish between the author's file and the content model, by another route.

This applies to any content model, not just compositions. A tree-owning rune with a narrow preamble loses unmatched content the same way, so the fix belongs in the content-model resolver, not in the composition layer. First measure how many shipped runes drop content today, so the change can be staged as a warning before an error if needed.