feat: add MultiPlateNester with sorting logic

Implements static MultiPlateNester.SortItems with BoundingBoxArea and Size sort orders, covered by two passing xUnit tests.

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

OpenNest.Engine/MultiPlateNester.cs

@@ -0,0 +1,35 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace OpenNest
+{
+    public static class MultiPlateNester
+    {
+        public static List<NestItem> SortItems(List<NestItem> items, PartSortOrder sortOrder)
+        {
+            switch (sortOrder)
+            {
+                case PartSortOrder.BoundingBoxArea:
+                    return items
+                        .OrderByDescending(i =>
+                        {
+                            var bb = i.Drawing.Program.BoundingBox();
+                            return bb.Width * bb.Length;
+                        })
+                        .ToList();
+
+                case PartSortOrder.Size:
+                    return items
+                        .OrderByDescending(i =>
+                        {
+                            var bb = i.Drawing.Program.BoundingBox();
+                            return System.Math.Max(bb.Width, bb.Length);
+                        })
+                        .ToList();
+
+                default:
+                    return items.ToList();
+            }
+        }
+    }
+}