Add dcy-003 (confirmed) + REQ-011~015: broker execution + KIS + outbox

dcy-003 confirmed (all OQ answered, operator-confirmed scope/risk):
- OQ-1: paper/stub + real KIS Open API adapter (validated on KIS paper
  server; live account go-live config-gated, deferred).
- OQ-2: unified async fill (submit -> poll); paper is a deterministic
  async double (no synchronous shortcut) for faithful path coverage.
- OQ-3: option A — pending slot is the execution queue, broker_order_id
  is the idempotency guard; no separate outbox table (option B as a
  growth hook; intent_log stays dedup-only).
- OQ-4: dedicated background execution loop (claim -> submit -> poll ->
  reconcile); intent API only transitions to pending.

Decompose into REQ-011~015 (all Proposed): BrokerPort + paper double,
KIS adapter, execution loop, durability+reconciliation (Exception),
baseline alignment. NFR-1/2/3 absorbed. Update SRS index + dcy-003
created-REQ refs. Safety floor, KIS live account, strategy daemons, and
auth remain deferred to follow-up Discoveries.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/discovery/index.md

@@ -7,4 +7,5 @@
 | ID | 날짜 | 이슈명 | 문서 | 상태 |
 |---|---|---|---|---|
 | dcy-001_20260603_slot-pure-domain-migration | 20260603 | trex-core 베이스 수립 및 slot 순수 도메인 이식 | [Discovery](./dcy-001_20260603_slot-pure-domain-migration.md) | confirmed |
-| dcy-002_20260603_slot-core-daemon-http-api | 20260603 | trex-core 슬롯 코어 데몬화 + HTTP intent API | [Discovery](./dcy-002_20260603_slot-core-daemon-http-api.md) | confirmed |
+| dcy-002_20260603_slot-core-daemon-http-api | 20260603 | trex-core 슬롯 코어 데몬화 + HTTP intent API | [Discovery](./dcy-002_20260603_slot-core-daemon-http-api.md) | confirmed |
+| dcy-003_20260604_broker-execution-and-outbox | 20260604 | 브로커 단일 세션 + 주문 집행 + durable outbox (불변식 B) | [Discovery](./dcy-003_20260604_broker-execution-and-outbox.md) | confirmed |