feat: add ISplitFeature interface and StraightSplit implementation

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

OpenNest.Core/Splitting/ISplitFeature.cs

@@ -0,0 +1,22 @@
+using System.Collections.Generic;
+using OpenNest.Geometry;
+
+namespace OpenNest;
+
+public class SplitFeatureResult
+{
+    public List<Entity> NegativeSideEdge { get; }
+    public List<Entity> PositiveSideEdge { get; }
+
+    public SplitFeatureResult(List<Entity> negativeSideEdge, List<Entity> positiveSideEdge)
+    {
+        NegativeSideEdge = negativeSideEdge;
+        PositiveSideEdge = positiveSideEdge;
+    }
+}
+
+public interface ISplitFeature
+{
+    string Name { get; }
+    SplitFeatureResult GenerateFeatures(SplitLine line, double extentStart, double extentEnd, SplitParameters parameters);
+}