feat: add SplitLine and SplitParameters models

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

OpenNest.Tests/Splitting/SplitLineTests.cs

@@ -0,0 +1,31 @@
+namespace OpenNest.Tests.Splitting;
+
+public class SplitLineTests
+{
+    [Fact]
+    public void SplitLine_Vertical_StoresPositionAsX()
+    {
+        var line = new SplitLine(50.0, CutOffAxis.Vertical);
+        Assert.Equal(50.0, line.Position);
+        Assert.Equal(CutOffAxis.Vertical, line.Axis);
+    }
+
+    [Fact]
+    public void SplitLine_Horizontal_StoresPositionAsY()
+    {
+        var line = new SplitLine(30.0, CutOffAxis.Horizontal);
+        Assert.Equal(30.0, line.Position);
+        Assert.Equal(CutOffAxis.Horizontal, line.Axis);
+    }
+
+    [Fact]
+    public void SplitParameters_Defaults()
+    {
+        var p = new SplitParameters();
+        Assert.Equal(SplitType.Straight, p.Type);
+        Assert.Equal(3, p.TabCount);
+        Assert.Equal(1.0, p.TabWidth);
+        Assert.Equal(0.125, p.TabHeight);
+        Assert.Equal(2, p.SpikePairCount);
+    }
+}