feat: add part classification (large/medium/small) to MultiPlateNester

Introduces PartClass enum and Classify() static method that categorizes
parts as Large (exceeds half work area in either dimension), Medium
(area > 1/9 work area), or Small.

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

OpenNest.Engine/MultiPlateNester.cs

@@ -1,8 +1,16 @@
 using System.Collections.Generic;
 using System.Linq;
+using OpenNest.Geometry;
 
 namespace OpenNest
 {
+    public enum PartClass
+    {
+        Large,
+        Medium,
+        Small,
+    }
+
     public static class MultiPlateNester
     {
         public static List<NestItem> SortItems(List<NestItem> items, PartSortOrder sortOrder)
@@ -31,5 +39,22 @@ namespace OpenNest
                     return items.ToList();
             }
         }
+
+        public static PartClass Classify(Box partBounds, Box workArea)
+        {
+            var halfWidth = workArea.Width / 2.0;
+            var halfLength = workArea.Length / 2.0;
+
+            if (partBounds.Width > halfWidth || partBounds.Length > halfLength)
+                return PartClass.Large;
+
+            var workAreaArea = workArea.Width * workArea.Length;
+            var partArea = partBounds.Width * partBounds.Length;
+
+            if (partArea > workAreaArea / 9.0)
+                return PartClass.Medium;
+
+            return PartClass.Small;
+        }
     }
 }