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>

tests/application/test_fee_tax_policy.py

@@ -0,0 +1,76 @@
+"""Fee/tax policy tests (REQ-018 CHG-20260612-01, dcy-026, issue #24).
+
+Validates the single fee/tax policy point (`calculate_fee`/`calculate_tax`)
+and the paper broker carrying the policy-computed sell transaction tax — the
+correction for the `tax=0.0` hardcoding that under-counted realized P&L.
+"""
+from __future__ import annotations
+
+from trex_core.adapters.broker.paper import PaperBroker
+from trex_core.application.broker_port import OrderRequest, calculate_fee, calculate_tax
+
+
+# --- AC-6/AC-7: policy functions -------------------------------------------
+
+def test_calculate_fee_same_rate_buy_and_sell():
+    # 0.015 % commission on both sides (TREX_KIS_FEE_BPS=1.5 default)
+    assert calculate_fee("buy", 1_000_000) == 150.0
+    assert calculate_fee("sell", 1_000_000) == 150.0
+
+
+def test_calculate_tax_sell_only():
+    # 0.2 % securities transaction tax on sells (TREX_KRX_TAX_SELL_BPS=20.0)
+    assert calculate_tax("sell", 1_000_000) == 2000.0
+    # buys are tax-free (KR equities)
+    assert calculate_tax("buy", 1_000_000) == 0.0
+
+
+def test_fee_and_tax_zero_for_nonpositive_amount():
+    assert calculate_fee("sell", 0) == 0.0
+    assert calculate_fee("buy", -100) == 0.0
+    assert calculate_tax("sell", 0) == 0.0
+    assert calculate_tax("sell", -100) == 0.0
+
+
+# --- AC-9: issue #24 four-fill regression (expected total 1,376.9원) --------
+
+def test_issue_24_four_fill_regression():
+    # (side, price) with qty=1 — from issue #24 evidence (2026-06-11).
+    fills = [
+        ("buy", 291_500),
+        ("sell", 301_000),
+        ("buy", 295_000),
+        ("sell", 298_500),
+    ]
+    total_fee = sum(calculate_fee(side, amount) for side, amount in fills)
+    total_tax = sum(calculate_tax(side, amount) for side, amount in fills)
+
+    assert round(total_fee, 1) == 177.9
+    assert round(total_tax, 1) == 1199.0
+    assert round(total_fee + total_tax, 1) == 1376.9
+
+
+# --- AC-7: paper broker carries policy tax (parity with KIS policy) ---------
+
+def _fill(broker, *, side, symbol, qty, price):
+    ack = broker.submit_order(
+        OrderRequest(slot_id="s1", side=side, symbol=symbol, qty=qty, limit_price=price)
+    )
+    fs = broker.poll_fill(ack.broker_order_id, side=side, symbol=symbol)
+    assert fs.is_filled
+    return fs
+
+
+def test_paper_sell_fill_carries_policy_tax():
+    broker = PaperBroker(fill_after_polls=0)
+    fs = _fill(broker, side="sell", symbol="005930", qty=1, price=301_000)
+    assert fs.fee == calculate_fee("sell", 301_000)
+    assert fs.tax == calculate_tax("sell", 301_000)
+    assert fs.tax > 0.0  # the bug: previously hardcoded to 0.0
+
+
+def test_paper_buy_fill_has_zero_tax():
+    broker = PaperBroker(fill_after_polls=0)
+    fs = _fill(broker, side="buy", symbol="005930", qty=1, price=291_500)
+    assert fs.fee == calculate_fee("buy", 291_500)
+    assert fs.tax == 0.0