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/CirclePacking/Item.cs

@@ -0,0 +1,19 @@
+using OpenNest.Geometry;
+
+namespace OpenNest.CirclePacking
+{
+    internal class Item : Circle
+    {
+        public int Id { get; set; }
+
+        public object Clone()
+        {
+            return new Item
+            {
+                Radius = this.Radius,
+                Center = this.Center,
+                Id = this.Id
+            };
+        }
+    }
+}