refactor: 新增 DirectionRatio 方向因子纯函数, 替换7处三元式

多空方向(PositionType==Long?1:-1)和收付方向(xxx==收取?1:-1)
在两个上帝类里重复 15+处, +1/-1 写反是常见 bug 源。

新增 ReturnLegs/DirectionRatio.cs:
- LongShort(positionType): 多头=+1, 空头=-1
- ReceivePay(direction): 收取=+1, 支付=-1

替换:
- SwapEodPositionService: 6处 shortRatio 三元式(含注释1处)
- SwapDealService: 1处 shortRatio + 1处 directionRatio

验证: 编译0错误, 全量511测试7失败(基线一致)。

UnitTestProject/Modules/SwapModule/ReturnLegs/DirectionRatioTest.cs

@@ -0,0 +1,27 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using YLErp.DBModels;
+using YLErp.DBModels.Enums;
+using YLErp.Modules.SwapModule.ReturnLegs;
+
+namespace UnitTestProject.Modules.SwapModule.ReturnLegs
+{
+    [TestClass]
+    public class DirectionRatioTest
+    {
+        [TestMethod]
+        public void LongShort_多头_返回1()
+            => Assert.AreEqual(1, DirectionRatio.LongShort((int)PositionTypeFlag.Long));
+
+        [TestMethod]
+        public void LongShort_空头_返回负1()
+            => Assert.AreEqual(-1, DirectionRatio.LongShort((int)PositionTypeFlag.Short));
+
+        [TestMethod]
+        public void ReceivePay_收取_返回1()
+            => Assert.AreEqual(1, DirectionRatio.ReceivePay((int)SwapDirectionEnum.收取));
+
+        [TestMethod]
+        public void ReceivePay_支付_返回负1()
+            => Assert.AreEqual(-1, DirectionRatio.ReceivePay((int)SwapDirectionEnum.支付));
+    }
+}