fix(reconcile): symbol을 슬롯 생성 시 함께 저장 + NULL 슬롯 backfill

reconcile_universe 시그니처를 set[str] → dict[str, str] (portfolio_id→symbol)로 변경.
기존 symbol=NULL 슬롯은 다음 reconcile 실행 시 자동 backfill.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

trex_core/domain/slot/in_memory_repository.py

@@ -84,7 +84,7 @@ class InMemorySlotRepository:
         ]
 
     def reconcile_universe(
-        self, instance_key: str, universe_portfolio_ids: set[str],
+        self, instance_key: str, universe: dict[str, str],
         *, allow_retire: bool = True,
     ) -> dict[str, int]:
         existing = {
@@ -93,13 +93,18 @@ class InMemorySlotRepository:
         }
         now = datetime.now(tz=timezone.utc)
         created = retired = tombstoned = 0
+        # backfill symbol on slots that were created before this field was populated
+        for s in self._slots.values():
+            if (s.instance_key == instance_key and s.portfolio_id in universe
+                    and s.symbol is None):
+                self._slots[s.slot_id] = replace(s, symbol=universe[s.portfolio_id], updated_at=now)
         # create asset slots for newly-appeared universe assets (idempotent)
-        for pid in universe_portfolio_ids:
+        for pid, symbol in universe.items():
             if pid not in existing:
                 sid = uuid4()
                 self._slots[sid] = Slot(
                     slot_id=sid, status=SlotState.INACTIVE,
-                    instance_key=instance_key, portfolio_id=pid,
+                    instance_key=instance_key, portfolio_id=pid, symbol=symbol,
                     created_at=now, updated_at=now,
                 )
                 created += 1
@@ -107,7 +112,7 @@ class InMemorySlotRepository:
         # mass-disappearance guard, dcy-020/OQ-2 D3 — caller passes allow_retire=False)
         if allow_retire:
             for pid, s in existing.items():
-                if pid in universe_portfolio_ids:
+                if pid in universe:
                     continue
                 if s.status in self._ACTIVE:
                     tombstoned += 1  # holding/pending — keep (safety-sell), no retire

trex_core/domain/slot/repository.py

@@ -66,13 +66,14 @@ class SlotRepository(Protocol):
         ...
 
     def reconcile_universe(
-        self, instance_key: str, universe_portfolio_ids: set[str],
+        self, instance_key: str, universe: dict[str, str],
         *, allow_retire: bool = True,
     ) -> dict[str, int]:
         """Reconcile the slot set to the portfolio-hub universe (dcy-020/REQ-058).
 
+        universe: {portfolio_id -> symbol} mapping from PortfolioItem.
         - new portfolio_id (no slot for instance) -> create an inactive asset slot
-          (idempotent; keyed by (instance_key, portfolio_id), permanent).
+          with symbol set (idempotent; keyed by (instance_key, portfolio_id), permanent).
         - slot whose portfolio_id left the universe AND is inactive AND not yet
           retired -> soft-retire (set retired_at).
         - slot that left the universe but is holding/pending -> tombstone (left