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>

tests/application/test_safety_sell_visibility.py

@@ -49,7 +49,7 @@ def _wire(**cfg_over):
 def _holding(svc, repo, *, symbol="005930", **fields):
     from uuid import uuid4
     pid = f"pf-{uuid4().hex[:8]}"
-    repo.reconcile_universe("inst-1", {pid})
+    repo.reconcile_universe("inst-1", {pid: "SYM"})
     slot = repo.find_slot_by_portfolio("inst-1", pid)
     p = svc.transit_slot(slot.slot_id, SlotState.INACTIVE, SlotState.PENDING_BUY,
                          reason="buy", actor="t", symbol=symbol, **fields)