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/RectanglePacking/BinConverter.cs

@@ -0,0 +1,86 @@
+using System.Collections.Generic;
+using OpenNest.Geometry;
+using OpenNest.Math;
+
+namespace OpenNest.RectanglePacking
+{
+    internal static class BinConverter
+    {
+        public static Bin CreateBin(Box area, double partSpacing)
+        {
+            var bin = new Bin
+            {
+                Location = area.Location,
+                Size = area.Size
+            };
+
+            bin.Width += partSpacing;
+            bin.Height += partSpacing;
+
+            return bin;
+        }
+
+        public static Item ToItem(NestItem item, double partSpacing, int id = 0)
+        {
+            var box = item.Drawing.Program.BoundingBox();
+
+            box.Width += partSpacing;
+            box.Height += partSpacing;
+
+            return new Item
+            {
+                Id = id,
+                Location = box.Location,
+                Size = box.Size
+            };
+        }
+
+        public static List<Item> ToItems(List<NestItem> items, double partSpacing, double plateArea)
+        {
+            var binItems = new List<Item>();
+
+            for (var i = 0; i < items.Count; i++)
+            {
+                var item = items[i];
+                var binItem = ToItem(item, partSpacing, i);
+
+                var maxQty = (int)System.Math.Floor(plateArea / binItem.Area());
+                var qty = item.Quantity < maxQty ? item.Quantity : maxQty;
+
+                for (var j = 0; j < qty; j++)
+                    binItems.Add(binItem.Clone() as Item);
+            }
+
+            return binItems;
+        }
+
+        public static List<Part> ToParts(Bin bin, List<NestItem> items)
+        {
+            var parts = new List<Part>();
+
+            foreach (var item in bin.Items)
+            {
+                var nestItem = items[item.Id];
+                var part = ToPart(item, nestItem.Drawing);
+                parts.Add(part);
+            }
+
+            return parts;
+        }
+
+        private static Part ToPart(Item item, Drawing dwg)
+        {
+            var part = new Part(dwg);
+
+            if (item.IsRotated)
+                part.Rotate(Angle.HalfPI);
+
+            var boundingBox = part.Program.BoundingBox();
+            var offset = item.Location - boundingBox.Location;
+
+            part.Offset(offset);
+
+            return part;
+        }
+    }
+}