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>

trex_core/application/messaging_handlers.py

@@ -171,6 +171,7 @@ def make_snapshot_rpc_handler(
     watchdog_store: Optional[WatchdogStore] = None,
     watchdog_timeout_sec: int = 120,
     killswitch_store=None,
+    ledger_outbox=None,
 ):
     """Build the RPC handler for `trex.snapshot.req` (REQ-028).
 
@@ -229,6 +230,7 @@ def make_snapshot_rpc_handler(
             )
             payload = envelope.to_dict()
             _attach_watchdog(payload, slot_repo, day, watchdog_store, watchdog_timeout_sec)
+            _apply_cash_estimate(payload, ledger_outbox)
             return RpcResponse(payload=payload)
 
         # --- BAT-007 legacy stub-shape path ---
@@ -259,6 +261,27 @@ def make_snapshot_rpc_handler(
     return handler
 
 
+def _apply_cash_estimate(payload, ledger_outbox) -> None:
+    """Overlay an estimated cash for assets with unsettled (pending) ledger
+    writes (dcy-018 / REQ-029, OQ-4). cash_balance += pending cash delta and the
+    asset is flagged `cash_estimated=true` until option-D write-back lands.
+    Best-effort: any failure leaves the response unchanged (NFR-1)."""
+    if ledger_outbox is None:
+        return
+    try:
+        deltas = ledger_outbox.pending_cash_delta()
+        if not deltas:
+            return
+        for a in payload.get("assets", []):
+            d = deltas.get(a.get("asset_id"))
+            if d:
+                base = a.get("cash_balance") or 0.0
+                a["cash_balance"] = base + d
+                a["cash_estimated"] = True
+    except Exception:  # noqa: BLE001 - estimate must never break the response
+        return
+
+
 def _attach_watchdog(payload, slot_repo, day, watchdog_store, timeout_sec) -> None:
     """Attach holding-slot watchdog state to a snapshot response (dcy-017/REQ-034).