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_ledger.py

@@ -33,7 +33,7 @@ def _wire(broker, *, ledger=None, clock=None):
         LedgerWriter(outbox, ledger or InMemoryLedger(), clock=clock)
         if clock else LedgerWriter(outbox, ledger or InMemoryLedger())
     )
-    repo.reconcile_universe("default", {"PF-1"})
+    repo.reconcile_universe("default", {"PF-1": "SYM"})
     return repo, svc, intents, execution, outbox, writer