refactor: extract responsibilities from NestEngine into focused classes

- Move BuildPairParts to BestFitResult.BuildParts() instance method
- Extract BinConverter (RectanglePacking) for Part/NestItem/Bin conversions
- Extract RotationAnalysis for FindBestRotation and FindHullEdgeAngles

NestEngine reduced from 484 to 287 lines — now purely orchestration,
strategy selection, and comparison logic.

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

OpenNest.Engine/BestFit/BestFitResult.cs

@@ -1,3 +1,7 @@
+using System.Collections.Generic;
+using OpenNest.Geometry;
+using OpenNest.Math;
+
 namespace OpenNest.Engine.BestFit
 {
     public class BestFitResult
@@ -25,6 +29,30 @@ namespace OpenNest.Engine.BestFit
         {
             get { return System.Math.Min(BoundingWidth, BoundingHeight); }
         }
+
+        public List<Part> BuildParts(Drawing drawing)
+        {
+            var part1 = Part.CreateAtOrigin(drawing);
+
+            var part2 = Part.CreateAtOrigin(drawing, Candidate.Part2Rotation);
+            part2.Location = Candidate.Part2Offset;
+            part2.UpdateBounds();
+
+            if (!OptimalRotation.IsEqualTo(0))
+            {
+                var pairBounds = ((IEnumerable<IBoundable>)new IBoundable[] { part1, part2 }).GetBoundingBox();
+                var center = pairBounds.Center;
+                part1.Rotate(-OptimalRotation, center);
+                part2.Rotate(-OptimalRotation, center);
+            }
+
+            var finalBounds = ((IEnumerable<IBoundable>)new IBoundable[] { part1, part2 }).GetBoundingBox();
+            var offset = new Vector(-finalBounds.Left, -finalBounds.Bottom);
+            part1.Offset(offset);
+            part2.Offset(offset);
+
+            return new List<Part> { part1, part2 };
+        }
     }
 
     public enum BestFitSortField