perf: use convex hull NFP to avoid Clipper2 union bottleneck

ConvexMinkowskiSum is O(n+m) with no boolean geometry ops.
The concave Minkowski path was doing triangulation + pairwise
sums + Clipper2 Union, which hung at 100% CPU for complex parts.

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

OpenNest.Core/Geometry/NoFitPolygon.cs

@@ -78,7 +78,7 @@ namespace OpenNest.Geometry
         /// edge vectors sorted by angle. O(n+m) where n and m are vertex counts.
         /// Both polygons must have CCW winding.
         /// </summary>
-        internal static Polygon ConvexMinkowskiSum(Polygon a, Polygon b)
+        public static Polygon ConvexMinkowskiSum(Polygon a, Polygon b)
         {
             var edgesA = GetEdgeVectors(a);
             var edgesB = GetEdgeVectors(b);