_build_result applied the noise-label filter to every item in the range,
including the ones the result matched on. A hit on a footnote or index
entry returned its section with the matched text removed, the clip anchor
could not find the evidence and fell back to a prefix window, and the
un-merge path rebuilt through the same filter.
Noise is now a set of positions computed once per group by
_noise_positions: noise-labelled items minus the matched ones.
_expand_outward and _build_result take that set instead of a flag, so the
matched item is kept in content and counted toward the budget.
Footnotes leave the noise set. They carry sources, cross-references and
clarifications, and docling attaches table and figure footnotes to the
table itself, so the filter was dropping part of the table. The noise set
is page_header, page_footer and document_index.
Refs #609
d97aa15af9