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/SplitLineIntersect.cs

@@ -0,0 +1,81 @@
+using OpenNest.Geometry;
+using OpenNest.Math;
+using System.Collections.Generic;
+
+namespace OpenNest;
+
+/// <summary>
+/// Static helpers for testing entity-splitline intersections.
+/// </summary>
+public static class SplitLineIntersect
+{
+    /// <summary>
+    /// Finds the intersection point between an entity and a split line.
+    /// Returns null if no intersection or the entity doesn't straddle the split line.
+    /// </summary>
+    public static Vector? FindIntersection(Entity entity, SplitLine sl)
+    {
+        if (!CrossesSplitLine(entity, sl))
+            return null;
+
+        var bbox = entity.BoundingBox;
+        var margin = 1.0;
+
+        // Create a line at the split position spanning the entity's bbox extent (with margin)
+        Line splitLine;
+
+        if (sl.Axis == CutOffAxis.Vertical)
+            splitLine = sl.ToLine(bbox.Bottom - margin, bbox.Top + margin);
+        else
+            splitLine = sl.ToLine(bbox.Left - margin, bbox.Right + margin);
+
+        switch (entity.Type)
+        {
+            case EntityType.Line:
+                var line = (Line)entity;
+                if (Intersect.Intersects(line, splitLine, out var pt))
+                    return pt;
+                return null;
+
+            case EntityType.Arc:
+                var arc = (Arc)entity;
+                if (Intersect.Intersects(arc, splitLine, out var pts))
+                    return pts.Count > 0 ? pts[0] : null;
+                return null;
+
+            default:
+                return null;
+        }
+    }
+
+    /// <summary>
+    /// Returns true if the entity's bounding box straddles the split line,
+    /// meaning it extends to both sides of the split position (not just touching).
+    /// </summary>
+    public static bool CrossesSplitLine(Entity entity, SplitLine sl)
+    {
+        var bbox = entity.BoundingBox;
+
+        if (sl.Axis == CutOffAxis.Vertical)
+            return bbox.Left < sl.Position - Tolerance.Epsilon
+                && bbox.Right > sl.Position + Tolerance.Epsilon;
+        else
+            return bbox.Bottom < sl.Position - Tolerance.Epsilon
+                && bbox.Top > sl.Position + Tolerance.Epsilon;
+    }
+
+    /// <summary>
+    /// Returns -1 if the point is below/left of the split line,
+    /// +1 if above/right, or 0 if on the line (within tolerance).
+    /// </summary>
+    public static int SideOf(Vector pt, SplitLine sl)
+    {
+        var value = sl.Axis == CutOffAxis.Vertical ? pt.X : pt.Y;
+        var diff = value - sl.Position;
+
+        if (System.Math.Abs(diff) <= Tolerance.Epsilon)
+            return 0;
+
+        return diff < 0 ? -1 : 1;
+    }
+}