feat(ledger): dcy-018 option D cash/position write-back + 추정 현금 (BAT-023)

ledger write 응답의 portfolio-hub 권위 cash/quantity 를 스냅샷에 write-back 해
매수/매도 직후 "수량 live·현금 stale" 비대칭 제거(option D 활성화). portfolio-hub#8
(*_after 응답) 동작 중 확인으로 그간 deferred 됐던 REQ-030 을 닫음.

거버넌스: 전부 change-req (신규 REQ 없음, dcy-018 OQ-3 a).
- REQ-017: record_trade → LedgerWriteOutcome{result, cash_balance_after,
  asset_quantity_after}. 어댑터가 응답 root flat *_after 파싱(new+duplicate, 누락 None)
- REQ-019: LedgerWriter 에 SnapshotStore 주입 → new+duplicate write-back
  (best-effort, *_after 절대값이라 멱등). LedgerOutbox.pending_cash_delta()
- REQ-029: 추정 현금 오버레이(_apply_cash_estimate, cash_estimated 라벨, OQ-4 i)
  + 슬롯 오버레이 유지(OQ-1 B). 미정산 outbox delta 기반, write-back 수렴
- REQ-030: option D 활성화 → Approved→Implemented

best-effort 격리(NFR-1): write-back/추정 실패가 ledger 멱등·청산 무차단.
배치 번호: 동시 커밋된 BAT-022(slot-state, dcy-016)와 충돌해 BAT-023 으로 배정.

검증: 단위 350 passed/35 skipped, 인프라 게이트(PG+RabbitMQ) 385 passed, ruff clean.
신규 test_ledger_option_d + adapter/PG 테스트. BAT-023 release-candidate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/batches/index.md

@@ -30,3 +30,4 @@
 | BAT-020 | standard | release-candidate | REQ-032 (CHG-20260608-01), REQ-022 (CHG-20260608-01), REQ-021 (CHG-20260608-02), REQ-034 (CHG-20260608-01), REQ-026 (CHG-20260608-01) | dcy-016 | [bat-020](./bat-020_20260608_operator-cli-rpc-commands/) |
 | BAT-021 | standard | release-candidate | REQ-034 (CHG-20260608-02), REQ-021 (CHG-20260608-03), REQ-037 (CHG-20260608-01), REQ-038 (CHG-20260608-01), REQ-039 (CHG-20260608-01), REQ-032 (CHG-20260608-02), REQ-042 (CHG-20260608-01) | dcy-017 | [bat-021](./bat-021_20260608_watchdog-ownership-visibility/) |
 | BAT-022 | standard | release-candidate | REQ-028 (CHG-20260608-01), REQ-032 (CHG-20260608-03) | dcy-016 | [bat-022](./bat-022_20260608_slot-state-snapshot-visibility/) |
+| BAT-023 | standard | release-candidate | REQ-017 (CHG-20260608-02), REQ-019 (CHG-20260608-02), REQ-029 (CHG-20260608-01), REQ-030 (CHG-20260608-01, Approved→Implemented) | dcy-018 | [bat-023](./bat-023_20260608_ledger-cash-position-writeback-option-d/) |