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/PackEngine.cs

@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+
+namespace OpenNest.RectanglePacking
+{
+    internal abstract class PackEngine
+    {
+        public PackEngine(Bin bin)
+        {
+            Bin = bin;
+        }
+
+        public Bin Bin { get; set; }
+
+        public abstract void Pack(List<Item> items);
+    }
+}