feat(slot): BAT-029 M2(a)/M3 — holding 불일치 reconcile + slots 단일 뷰 (dcy-020/REQ-062·063)

- REQ-062 divergence: list_holding_slots(protocol+in_memory+PG) +
  reconcile_core_holdings(A=코어 holding/broker0→error, C=수량 불일치→warning,
  B=broker 보유/코어 없음→warning; 자동 청산 없음 issue #15). snapshot_scheduler
  자산 중심 모드에서 best-effort 호출, daemon slot_service 배선.
- REQ-063: slot_to_dict 에 universe_detached(retired_at) 추가 — slots 단일 뷰 tombstone 표시.
- 신규 테스트: test_holding_divergence(4).

M2(b) 청산 수량 브로커 정본(execution dispatch)은 라이브 집행 변경 → BAT-δ carry-over.

검증: 단위 393 passed / 36 skipped, PG-gated 18 passed, ruff clean. legacy 무파손.
REQ-061/062/063 Approved 유지(γ 경로 도입 검증; default 전환·M2(b) 는 BAT-δ).

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

trex_core/adapters/persistence/postgresql.py

@@ -154,6 +154,16 @@ class PostgreSQLSlotRepository:
                 row = cur.fetchone()
                 return int(row[0]) if row else 0
 
+    def list_holding_slots(self, instance_key: str) -> list[Slot]:
+        with self._psycopg.connect(self._dsn) as conn:
+            with conn.cursor() as cur:
+                cur.execute(
+                    f"SELECT {_SLOT_COLUMNS} FROM daily_slot "
+                    "WHERE instance_key = %s AND status = 'holding'",
+                    (instance_key,),
+                )
+                return [self._row_to_slot(r) for r in cur.fetchall()]
+
     def reconcile_universe(
         self, instance_key: str, trading_date: date, universe_portfolio_ids: set[str],
         *, allow_retire: bool = True,