feat: add SplitLine and SplitParameters models

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

OpenNest.Core/Splitting/SplitLine.cs

@@ -0,0 +1,17 @@
+namespace OpenNest;
+
+/// <summary>
+/// Defines a split line at a position along an axis.
+/// For Vertical, Position is the X coordinate. For Horizontal, Position is the Y coordinate.
+/// </summary>
+public class SplitLine
+{
+    public double Position { get; }
+    public CutOffAxis Axis { get; }
+
+    public SplitLine(double position, CutOffAxis axis)
+    {
+        Position = position;
+        Axis = axis;
+    }
+}