fix: flip ComputeNormal for CCW arcs on concave contour features

CCW arcs (e.g. the top of a U-slot) had the radial normal pointing
into the part material instead of into the scrap. This caused the
lead-in preview to flip sides on concave features.

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

OpenNest.Core/CNC/CuttingStrategy/ContourCuttingStrategy.cs

@@ -169,6 +169,12 @@ namespace OpenNest.CNC.CuttingStrategy
             {
                 // Radial direction from center to point
                 normal = point.AngleFrom(arc.Center);
+
+                // For CCW arcs the radial points the wrong way — flip it.
+                // CW arcs are convex features (corners) where radial = outward.
+                // CCW arcs are concave features (slots) where radial = inward.
+                if (arc.Rotation == RotationType.CCW)
+                    normal += System.Math.PI;
             }
             else if (entity is Circle circle)
             {