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/CNC/Kerf.cs

@@ -0,0 +1,39 @@
+namespace OpenNest.CNC
+{
+    public class Kerf : ICode
+    {
+        public Kerf(KerfType kerf = KerfType.Left)
+        {
+            Value = kerf;
+        }
+
+        public KerfType Value { get; set; }
+
+        public CodeType Type
+        {
+            get { return CodeType.SetKerf; }
+        }
+
+        public ICode Clone()
+        {
+            return new Kerf(Value);
+        }
+
+        public override string ToString()
+        {
+            switch (Value)
+            {
+                case KerfType.Left:
+                    return "G41";
+
+                case KerfType.Right:
+                    return "G42";
+
+                case KerfType.None:
+                    return "G40";
+            }
+
+            return string.Empty;
+        }
+    }
+}