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/Geometry/Layer.cs

@@ -0,0 +1,29 @@
+using System.Drawing;
+
+namespace OpenNest.Geometry
+{
+    public class Layer
+    {
+        public static readonly Layer Default = new Layer("0")
+        {
+            Color = Color.White,
+            IsVisible = true
+        };
+
+        public Layer(string name)
+        {
+            Name = name;
+        }
+
+        public string Name { get; set; }
+
+        public bool IsVisible { get; set; }
+
+        public Color Color { get; set; }
+
+        public override string ToString()
+        {
+            return Name;
+        }
+    }
+}