feat(slot): dcy-021 포지션 사이징 — 보유 슬롯 증액·부분 매도 (BAT-031)

자산 영구 슬롯(dcy-020)의 수량 가변 전제 위에 보유 포지션 수량 조정 구현.
명령형 계약(전략이 cash+quantity 로 qty+price 계산), core 는 실행 substrate,
수량 정본 = 브로커 잔고 재동기화로 "주문 수량 vs 보유 수량" 충돌 해소(별도 필드 없음).

- state.py: holding→pending_buy 전이 추가(증액) → 13 transitions (REQ-057)
- service.py: PENDING_SELL 진입 qty=caller as-is(None=전량 sentinel, 값=부분 주문) (REQ-062)
- intents.py: 증액 admission(holding 슬롯 매수=increase, M 캡 미적용), SellIntent.qty 부분 매도 (REQ-061)
- execution.py: _balance_map; 매도 dispatch=브로커잔고(전량)/min(주문,잔고)(부분);
  매수 체결 avg_price 가중평균+브로커 정본 resync; 매도 체결 잔고>0→holding/0→inactive;
  증액 롤백 holding 복귀 (REQ-066/062)
- messaging_handlers.py: sell payload qty 파싱(부분 매도)
- baseline 4문서(data-model/runtime-flows/interface-contract/project-structure) 정합 (REQ-067)

검증: 단위 378 / 통합(PG+RabbitMQ) 422 passed, ruff clean. run-the-app 사이징 e2e:
매수 10@1000 → 증액 +5@1200 → 15@1066.67(가중평균) → 부분 매도 6 → 9@1066.67(cost basis) → 전량→inactive.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

trex_core/application/intents.py

@@ -78,6 +78,9 @@ class SellIntent:
     # dcy-012 / REQ-007: optional sell limit price. None => market sell
     # (the path used by safety-sell / kill-switch). Set => limit sell at price.
     limit_price: Optional[float] = None
+    # dcy-021 / REQ-062: optional partial-sell quantity. None => sell the whole
+    # position (broker balance); set => partial sell of min(qty, broker balance).
+    qty: Optional[float] = None
     extra: dict[str, Any] = field(default_factory=dict)
 
 
@@ -140,6 +143,7 @@ class IntentService:
             "slot_id": str(intent.slot_id),
             "reason": intent.reason,
             "sell_strategy_id": intent.sell_strategy_id,
+            "qty": intent.qty,
         }
         prior = self._dedup(intent.intent_id, "sell", payload)
         if prior is not None:
@@ -163,15 +167,17 @@ class IntentService:
         return IntentDecision(intent_id, rec.status, rec.slot_id, rec.reason)
 
     def _admit_buy(self, intent: BuyIntent) -> IntentDecision:
-        """dcy-020 / REQ-059·REQ-061: activate the asset's permanent slot.
+        """dcy-020 / REQ-059·REQ-061 + dcy-021/REQ-066: activate or size up the
+        asset's permanent slot.
 
         Contract key is slot_id (REQ-061); portfolio_id is accepted as a legacy
         dual-accept mapping. Symbol is resolved from the slot's portfolio_id.
-        FCFS + no eviction: already-active -> already_active; at M ->
-        capacity_full (existing holdings untouched, incl. over-cap). Single
-        intent thread + instance-scoped advisory lock serialize admissions
-        (dcy-020 OQ-1)."""
-        _ACTIVE = (SlotState.PENDING_BUY, SlotState.HOLDING, SlotState.PENDING_SELL)
+        - INACTIVE slot -> initial buy (FCFS, M cap applies, no eviction).
+        - HOLDING slot  -> position-sizing INCREASE (dcy-021): adds to the held
+          position; M cap does NOT apply (active asset count unchanged).
+        - PENDING_* slot -> in-flight conflict (already_active).
+        Single intent thread + instance-scoped advisory lock serialize
+        admissions (dcy-020 OQ-1)."""
 
         def _resolve(sl):
             pid = sl.portfolio_id
@@ -191,12 +197,20 @@ class IntentService:
                 return IntentDecision(intent.intent_id, "rejected", None, "slot_id_or_portfolio_required")
             if slot is None:
                 return IntentDecision(intent.intent_id, "rejected", None, miss_reason)
-            if slot.status in _ACTIVE:
+            # dcy-021: a buy on a HOLDING slot is a position-sizing increase.
+            is_increase = slot.status is SlotState.HOLDING
+            if slot.status in (SlotState.PENDING_BUY, SlotState.PENDING_SELL):
                 return IntentDecision(intent.intent_id, "rejected", None, "already_active")
+            if slot.status not in (SlotState.INACTIVE, SlotState.HOLDING):
+                return IntentDecision(
+                    intent.intent_id, "rejected", None, f"slot_not_buyable:{slot.status.value}"
+                )
             symbol = _resolve(slot)
             if not symbol:
                 return IntentDecision(intent.intent_id, "rejected", None, "unknown_portfolio")
-            if (self._max_active_slots is not None
+            # M cap applies only to NEW activations (initial buy). An increase on
+            # an already-active asset does not change the active asset count.
+            if (not is_increase and self._max_active_slots is not None
                     and self._repo.count_active(self._instance_key) >= self._max_active_slots):
                 return IntentDecision(intent.intent_id, "rejected", None, "capacity_full")
             try:
@@ -204,7 +218,7 @@ class IntentService:
                     slot.slot_id,
                     slot.status,
                     SlotState.PENDING_BUY,
-                    reason="buy_intent",
+                    reason="position_increase" if is_increase else "buy_intent",
                     actor="intent_api",
                     symbol=symbol,
                     qty=intent.qty,
@@ -243,6 +257,10 @@ class IntentService:
                 sell_strategy_id=intent.sell_strategy_id,
                 # dcy-012 / REQ-007: optional sell limit (None => market sell).
                 limit_price=intent.limit_price,
+                # dcy-021 / REQ-062: partial-sell order qty (None => sell all);
+                # dispatch caps at the broker balance. Entering PENDING_SELL takes
+                # this qty as-is (None = "sell all" sentinel).
+                qty=intent.qty,
                 extra={"intent_id": intent.intent_id},
             )
         except SlotStateMismatchError: