buildDocument was a ~30-line method mixing attribution routing, date
parsing, authoritative collection management, file metadata, and
computed flags. Split into five named helpers — applyAttribution,
applyDates, applyAuthoritativeAssociations, applyFileMetadata,
applyComputedFlags — each doing one job. Pure refactor; all 43 existing
DocumentImporterTest cases still pass.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>