feat(broker): BAT-038 — 매도 증권거래세 누락 정정 + fee/tax 단일 정책 지점 (dcy-026/REQ-018, issue #24)

dcy-026 confirm-discovery + REQ-018 change-req(CHG-20260612-01)→구현→
confirm-req-implemented(CHG-20260612-02) 전체 delivery.

- broker_port.py: calculate_tax(side, amount) 신규 + fee/tax 율 env 상수화
  (TREX_KIS_FEE_BPS=1.5, TREX_KRX_TAX_SELL_BPS=20.0). 매도 0.2%/매수 0.
- paper.py / kis.py: tax=0.0 하드코딩 3곳 제거 → 정책 함수 호출.
  KIS daily-ccld 응답에 fee/tax 필드 부재 → 정책 폴백(향후 정본 우선 훅).
- runtime-flows.md: 체결→ledger fee/tax 산출 흐름 갱신.
- tests: fee/tax 정책 단위 + issue #24 4건 회귀(fee+tax=1,376.9원).
  429 passed (+6), ruff clean.

REQ-018 Approved→Implemented, BAT-038 release-candidate.

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

trex_core/adapters/broker/paper.py

@@ -17,6 +17,7 @@ from trex_core.application.broker_port import (
     OrderAck,
     OrderRequest,
     calculate_fee,
+    calculate_tax,
 )
 
 
@@ -84,10 +85,12 @@ class PaperBroker:
                 # Deterministic execution_id (dcy-004 OQ-3):
                 #   paper:{broker_order_id}:{filled_qty}
                 execution_id = f"paper:{o.oid}:{o.filled}"
-                fee = calculate_fee(o.side, o.filled * o.price)
+                amount = o.filled * o.price
+                fee = calculate_fee(o.side, amount)
+                tax = calculate_tax(o.side, amount)
                 return FillStatus(
                     state="filled", filled_qty=o.filled, avg_price=o.price,
-                    execution_id=execution_id, fee=fee, tax=0.0,
+                    execution_id=execution_id, fee=fee, tax=tax,
                 )
             return FillStatus(state="partially_filled", filled_qty=o.filled, avg_price=o.price)