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/SpecialLayers.cs

@@ -0,0 +1,21 @@
+using OpenNest.Geometry;
+
+namespace OpenNest
+{
+    public static class SpecialLayers
+    {
+        public static readonly Layer Default = new Layer("0");
+
+        public static readonly Layer Cut = new Layer("CUT");
+
+        public static readonly Layer Rapid = new Layer("RAPID");
+
+        public static readonly Layer Display = new Layer("DISPLAY");
+
+        public static readonly Layer Leadin = new Layer("LEADIN");
+
+        public static readonly Layer Leadout = new Layer("LEADOUT");
+
+        public static readonly Layer Scribe = new Layer("SCRIBE");
+    }
+}