fix: derive multi-day trades from target-weight deltas

tests/test_execution.py

@@ -329,12 +329,18 @@ def test_simulate_multi_day_length_mismatch_raises():
 
 
 def test_simulate_multi_day_first_day_value_equals_initial():
-    """第一天 portfolio_value = initial_cash（无持仓）。"""
+    """零成本下第一天日末 NAV 等于初始资金。"""
     signals = [("d1", {"A": 1.0})]
     prices = [("d1", {"A": 10.0})]
-    positions = simulate_multi_day(signals, prices, 1_000_000.0)
-    # 第一天 NAV = 1_000_000（无持仓），第二天才是调仓后
+    config = ExecutionConfig(
+        commission_bps=0,
+        stamp_tax_bps=0,
+        slippage_bps=0,
+        min_trade_amount=0,
+    )
+    positions = simulate_multi_day(signals, prices, 1_000_000.0, config)
     assert positions[0].portfolio_value == 1_000_000.0
+    assert positions[0].holdings == {"A": 100_000.0}
 
 
 def test_simulate_multi_day_holdings_evolution():
@@ -810,8 +816,8 @@ def test_compute_realized_pnl_sell_realizes():
         target_weights_history=targets,
     )
     pnl_list = compute_realized_pnl(positions)
-    # 第三天（卖出兑现）应有 realized 正利润（cash 从 -800 → 2M = +2M）
-    assert pnl_list[2].realized_pnl > 0
+    # 第二天日末快照已包含当日卖出，现金流入应在当天反映。
+    assert pnl_list[1].realized_pnl > 0
 
 
 # ── O3: end-to-end 端到端测试（集成多个函数） ──────────────