refactor(engine): precompute hull angles during pair evaluation

Store hull edge angles in BestFitResult at evaluation time so they
don't need to be recomputed during the fill phase. Extract
GetHullEdgeAngles(Polygon) overload from FindHullEdgeAngles.

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

OpenNest.Engine/RotationAnalysis.cs

@@ -80,6 +80,11 @@ namespace OpenNest
                 return new List<double> { 0 };
 
             var hull = ConvexHull.Compute(points);
+            return GetHullEdgeAngles(hull);
+        }
+
+        public static List<double> GetHullEdgeAngles(Polygon hull)
+        {
             var vertices = hull.Vertices;
             var n = hull.IsClosed() ? vertices.Count - 1 : vertices.Count;