DR-051 — Per-grain "content date" anchors for corpus search date filtering

Retired/tombstoned S504. Re-homed (S504 register sweep): behaviour is pinned by a test gate at the action site. Substance home: canonical:tests/integration/hybrid-search-completion.integration.test.ts + app/api/search/route.ts. Original ADR text retained below; this id stays on disk so citations resolve and the number is never re-issued. The server-side date-range filter anchors on: source_documents/content_chunks → captured_date; q_a_pairs → valid_from; reference_items → published_at — with STRICT-EXCLUDE NULL handling (rows without the anchor date are excluded while a date filter is active; a filtered result set never contains undated rows).