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/IPostProcessor.cs

@@ -0,0 +1,17 @@
+using System.IO;
+
+namespace OpenNest
+{
+    public interface IPostProcessor
+    {
+        string Name { get; }
+
+        string Author { get; }
+
+        string Description { get; }
+
+        void Post(Nest nest, Stream outputStream);
+
+        void Post(Nest nest, string outputFile);
+    }
+}