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_execution.py

@@ -24,7 +24,7 @@ def _wire(broker, max_active_slots=None):
         instance_key=INST, max_active_slots=max_active_slots,
     )
     execution = ExecutionService(svc, repo, broker, instance_key=INST)
-    repo.reconcile_universe(INST, set(SYMBOLS))  # permanent asset slots
+    repo.reconcile_universe(INST, SYMBOLS)  # permanent asset slots
     return repo, svc, intents, execution