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/RelativePosition.cs

@@ -0,0 +1,13 @@
+
+namespace OpenNest
+{
+    public enum RelativePosition
+    {
+        Intersecting,
+        Left,
+        Right,
+        Top,
+        Bottom,
+        None
+    }
+}