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>

tests/application/test_ledger_outbox_backoff.py

@@ -19,6 +19,7 @@ from trex_core.application.ledger_port import (
     LedgerPermanentError,
     LedgerRetryableError,
     LedgerTrade,
+    LedgerWriteOutcome,
     LedgerWriteResult,
 )
 
@@ -40,7 +41,7 @@ class _ProgrammableLedger:
         self.behavior = behavior
         self.calls = 0
 
-    def record_trade(self, trade) -> LedgerWriteResult:
+    def record_trade(self, trade) -> LedgerWriteOutcome:
         self.calls += 1
         return self.behavior(trade)
 
@@ -142,7 +143,7 @@ def test_backoff_row_does_not_block_new_rows():
     def behavior(trade):
         if trade.asset_id == "PF-bad":
             raise LedgerRetryableError("503")
-        return LedgerWriteResult.NEW
+        return LedgerWriteOutcome(LedgerWriteResult.NEW)
 
     outbox, ledger, writer = _wire(
         behavior, clock=lambda: now["t"], retry_base_sec=100.0, max_attempts=99,