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.Core/Collections/DrawingCollection.cs

@@ -0,0 +1,8 @@
+using System.Collections.Generic;
+
+namespace OpenNest.Collections
+{
+    public class DrawingCollection : HashSet<Drawing>
+    {
+    }
+}