feat: add ISplitFeature interface and StraightSplit implementation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest.Tests/Splitting/SplitFeatureTests.cs

@@ -0,0 +1,52 @@
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests.Splitting;
+
+public class SplitFeatureTests
+{
+    [Fact]
+    public void StraightSplit_Vertical_ProducesSingleLineEachSide()
+    {
+        var feature = new StraightSplit();
+        var line = new SplitLine(50.0, CutOffAxis.Vertical);
+        var parameters = new SplitParameters { Type = SplitType.Straight };
+
+        var result = feature.GenerateFeatures(line, 10.0, 90.0, parameters);
+
+        Assert.Single(result.NegativeSideEdge);
+        var negLine = Assert.IsType<Line>(result.NegativeSideEdge[0]);
+        Assert.Equal(50.0, negLine.StartPoint.X, 6);
+        Assert.Equal(10.0, negLine.StartPoint.Y, 6);
+        Assert.Equal(50.0, negLine.EndPoint.X, 6);
+        Assert.Equal(90.0, negLine.EndPoint.Y, 6);
+
+        Assert.Single(result.PositiveSideEdge);
+        var posLine = Assert.IsType<Line>(result.PositiveSideEdge[0]);
+        Assert.Equal(50.0, posLine.StartPoint.X, 6);
+        Assert.Equal(90.0, posLine.StartPoint.Y, 6);
+        Assert.Equal(50.0, posLine.EndPoint.X, 6);
+        Assert.Equal(10.0, posLine.EndPoint.Y, 6);
+    }
+
+    [Fact]
+    public void StraightSplit_Horizontal_ProducesSingleLineEachSide()
+    {
+        var feature = new StraightSplit();
+        var line = new SplitLine(40.0, CutOffAxis.Horizontal);
+        var parameters = new SplitParameters { Type = SplitType.Straight };
+
+        var result = feature.GenerateFeatures(line, 5.0, 95.0, parameters);
+
+        var negLine = Assert.IsType<Line>(result.NegativeSideEdge[0]);
+        Assert.Equal(5.0, negLine.StartPoint.X, 6);
+        Assert.Equal(40.0, negLine.StartPoint.Y, 6);
+        Assert.Equal(95.0, negLine.EndPoint.X, 6);
+        Assert.Equal(40.0, negLine.EndPoint.Y, 6);
+    }
+
+    [Fact]
+    public void StraightSplit_Name()
+    {
+        Assert.Equal("Straight", new StraightSplit().Name);
+    }
+}