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();
+            }
+        }
+    }
+}

OpenNest.Tests/Engine/MultiPlateNesterTests.cs

@@ -0,0 +1,65 @@
+using OpenNest.Geometry;
+using System.Collections.Generic;
+using System.Linq;
+using Xunit;
+
+namespace OpenNest.Tests.Engine;
+
+public class MultiPlateNesterTests
+{
+    private static Drawing MakeDrawing(string name, double width, double length)
+    {
+        var program = new OpenNest.CNC.Program();
+        program.Codes.Add(new OpenNest.CNC.RapidMove(new Vector(0, 0)));
+        program.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(width, 0)));
+        program.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(width, length)));
+        program.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(0, length)));
+        program.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(0, 0)));
+        var drawing = new Drawing(name, program);
+        drawing.UpdateArea();
+        return drawing;
+    }
+
+    private static NestItem MakeItem(string name, double width, double length, int qty = 1)
+    {
+        return new NestItem
+        {
+            Drawing = MakeDrawing(name, width, length),
+            Quantity = qty,
+        };
+    }
+
+    [Fact]
+    public void SortByBoundingBoxArea_OrdersLargestFirst()
+    {
+        var items = new List<NestItem>
+        {
+            MakeItem("small", 10, 10),
+            MakeItem("large", 40, 60),
+            MakeItem("medium", 20, 30),
+        };
+
+        var sorted = MultiPlateNester.SortItems(items, PartSortOrder.BoundingBoxArea);
+
+        Assert.Equal("large", sorted[0].Drawing.Name);
+        Assert.Equal("medium", sorted[1].Drawing.Name);
+        Assert.Equal("small", sorted[2].Drawing.Name);
+    }
+
+    [Fact]
+    public void SortBySize_OrdersByLongestDimension()
+    {
+        var items = new List<NestItem>
+        {
+            MakeItem("short-wide", 50, 20),    // longest = 50
+            MakeItem("tall-narrow", 10, 80),    // longest = 80
+            MakeItem("square", 30, 30),         // longest = 30
+        };
+
+        var sorted = MultiPlateNester.SortItems(items, PartSortOrder.Size);
+
+        Assert.Equal("tall-narrow", sorted[0].Drawing.Name);
+        Assert.Equal("short-wide", sorted[1].Drawing.Name);
+        Assert.Equal("square", sorted[2].Drawing.Name);
+    }
+}