修复补减仓时对原持仓重复计入交易滑点

crates/fidc-core/src/stock_pool_execution.rs

@@ -1588,7 +1588,17 @@ pub fn build_stock_pool_target_plan_with_fee_model(
         if sizing_price <= Decimal::ZERO {
             return Err(format!("{symbol} execution sizing price is invalid"));
         }
-        let raw_target = (target_value / sizing_price).floor();
+        // Existing shares are marked at the observed market price. Only the
+        // new buy leg pays its executable/slippage price; repricing the whole
+        // position would charge fictitious slippage and miss a board lot.
+        let current_value = current_quantity * quote.last_price;
+        let raw_target = if target_value >= current_value {
+            current_quantity + ((target_value - current_value) / sizing_price).floor()
+        } else {
+            // Sale slippage changes proceeds, not the marked shares we must
+            // remove to reach a market-value target.
+            (target_value / quote.last_price).floor()
+        };
         let (step, minimum_buy) = order_quantity_rules(quote)?;
         let mut target_quantity = current_quantity;
         let mut delta = Decimal::ZERO;
@@ -1911,7 +1921,7 @@ pub fn build_stock_pool_target_plan_with_fee_model(
         let cost = |quantity: Decimal| {
             Ok(quantity * price + fee_for(&row.symbol, OrderSide::Buy, quantity * price)?)
         };
-        let own_budget = (row.target_value - row.current_quantity * price).max(Decimal::ZERO);
+        let own_budget = (row.target_value - row.current_quantity * quote.last_price).max(Decimal::ZERO);
         let allocation_quantity = max_affordable_buy_quantity_with_cost(
             own_budget,
             row.delta_quantity,