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/application/snapshot_scheduler.py

@@ -128,7 +128,11 @@ class SnapshotScheduler:
 
     # ---- asset-centric slot reconcile (dcy-020/REQ-058, BAT-028) ----------
     def _reconcile_slots(self, items: list) -> None:
-        universe = {it.portfolio_id for it in items if getattr(it, "portfolio_id", None)}
+        universe = {
+            it.portfolio_id: it.symbol
+            for it in items
+            if getattr(it, "portfolio_id", None)
+        }
         if not universe:
             _log.warning("slot reconcile skipped: empty universe (degrade guard)")
             return