Restructure project layout to flatten directory structure

Move all projects from Source/ to repository root for simpler navigation.
- Remove External/ dependency DLLs (will use NuGet packages)
- Remove Installer/ NSIS script
- Replace PartCollection/PlateCollection with ObservableList
- Add packages.config for NuGet dependencies

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

OpenNest.Engine/RectanglePacking/Item.cs

@@ -0,0 +1,52 @@
+using System.Collections.Generic;
+
+namespace OpenNest.RectanglePacking
+{
+    internal class Item : Box
+    {
+        public int Id { get; set; }
+
+        public bool IsRotated { get; private set; }
+
+        public void Rotate()
+        {
+            Generic.Swap(ref Size.Width, ref Size.Height);
+            IsRotated = !IsRotated;
+        }
+
+        public object Clone()
+        {
+            return new Item
+            {
+                IsRotated = this.IsRotated,
+                Location = this.Location,
+                Size = this.Size,
+                Id = this.Id
+            };
+        }
+    }
+
+    internal static class ItemListExtensions
+    {
+        public static Box GetBoundingBox(this IList<Item> items)
+        {
+            if (items.Count == 0)
+                return Box.Empty;
+
+            double minX = items[0].X;
+            double minY = items[0].Y;
+            double maxX = items[0].X + items[0].Width;
+            double maxY = items[0].Y + items[0].Height;
+
+            foreach (var box in items)
+            {
+                if (box.Left < minX) minX = box.Left;
+                if (box.Right > maxX) maxX = box.Right;
+                if (box.Bottom < minY) minY = box.Bottom;
+                if (box.Top > maxY) maxY = box.Top;
+            }
+
+            return new Box(minX, minY, maxX - minX, maxY - minY);
+        }
+    }
+}