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/FillEngine.cs

@@ -0,0 +1,17 @@
+
+namespace OpenNest.CirclePacking
+{
+    internal abstract class FillEngine
+    {
+        public FillEngine(Bin bin)
+        {
+            Bin = bin;
+        }
+
+        public Bin Bin { get; set; }
+
+        public abstract void Fill(Item item);
+
+        public abstract void Fill(Item item, int maxCount);
+    }
+}