DR-092 — `extracted_from_corpus` provenance reads the extraction, not a derived path

Retired/tombstoned S504. Re-homed (S504 register sweep): zero canonical citations; the live half already sits at the action site. Substance home: canonical:app/api/q-a-pairs/[id]/route.ts:22 + tests/integration/cocoindex/platform-corpus-shape.test.ts:159. Original ADR text retained below; this id stays on disk so citations resolve and the number is never re-issued. q_a_pairs.source_document_id for corpus-promoted pairs is extraction.source_document_id — the real corpus document. The sdUuid5(qaSidecarRelPath(pairId)) derivation is retired: it minted a pointer to a source_documents row that has not existed since the file emit died at {131.8}, breaking four consumers silently. __qa__ remains a reserved prefix and the RATIFY-2 guard stays.