For any page name, the set of pages that link to it: extract wikilinks from every
union page (new UnionGraph.iter_pages enumeration) and index the resolved ones by
target name. Red-links create no backlinks; entries carry source provenance; a
chorus target aggregates the backlinks of all members under one name. Derived/
recomputable, stores nothing canonical.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
