feat: add SplitLineIntersect helper for entity-splitline intersection

Add ToLine() to SplitLine and create SplitLineIntersect static class with
FindIntersection, CrossesSplitLine, and SideOf methods for testing entity
intersections against split lines. These helpers support the upcoming
Clipper2-free DrawingSplitter rewrite.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Core/Splitting/SplitLine.cs

@@ -1,3 +1,4 @@
+using OpenNest.Geometry;
 using System.Collections.Generic;
 
 namespace OpenNest;
@@ -23,4 +24,16 @@ public class SplitLine
         Position = position;
         Axis = axis;
     }
+
+    /// <summary>
+    /// Returns a Line entity at the split position spanning the given extent range.
+    /// For Vertical: line from (Position, extentStart) to (Position, extentEnd).
+    /// For Horizontal: line from (extentStart, Position) to (extentEnd, Position).
+    /// </summary>
+    public Line ToLine(double extentStart, double extentEnd)
+    {
+        return Axis == CutOffAxis.Vertical
+            ? new Line(Position, extentStart, Position, extentEnd)
+            : new Line(extentStart, Position, extentEnd, Position);
+    }
 }