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;
+        }
     }
 }

OpenNest.Tests/Engine/MultiPlateNesterTests.cs

@@ -62,4 +62,45 @@ public class MultiPlateNesterTests
         Assert.Equal("short-wide", sorted[1].Drawing.Name);
         Assert.Equal("square", sorted[2].Drawing.Name);
     }
+
+    // --- Task 4: Part Classification ---
+
+    [Fact]
+    public void Classify_LargePart_WhenWidthExceedsHalfWorkArea()
+    {
+        var workArea = new Box(0, 0, 96, 48);
+        var bb = new Box(0, 0, 50, 20); // width 50 > half of 96 = 48
+        var result = MultiPlateNester.Classify(bb, workArea);
+        Assert.Equal(PartClass.Large, result);
+    }
+
+    [Fact]
+    public void Classify_LargePart_WhenLengthExceedsHalfWorkArea()
+    {
+        var workArea = new Box(0, 0, 96, 48);
+        var bb = new Box(0, 0, 20, 30); // length 30 > half of 48 = 24
+        var result = MultiPlateNester.Classify(bb, workArea);
+        Assert.Equal(PartClass.Large, result);
+    }
+
+    [Fact]
+    public void Classify_MediumPart_NotLargeButAreaAboveThreshold()
+    {
+        var workArea = new Box(0, 0, 96, 48);
+        // workArea = 4608, 1/9 = 512. bb = 40*15 = 600 > 512
+        // 40 < 48 (half of 96), 15 < 24 (half of 48) — not Large
+        var bb = new Box(0, 0, 40, 15);
+        var result = MultiPlateNester.Classify(bb, workArea);
+        Assert.Equal(PartClass.Medium, result);
+    }
+
+    [Fact]
+    public void Classify_SmallPart()
+    {
+        var workArea = new Box(0, 0, 96, 48);
+        // workArea = 4608, 1/9 = 512. bb = 10*10 = 100 < 512
+        var bb = new Box(0, 0, 10, 10);
+        var result = MultiPlateNester.Classify(bb, workArea);
+        Assert.Equal(PartClass.Small, result);
+    }
 }