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/Document.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using OpenNest.IO;
+
+namespace OpenNest
+{
+    public class Document
+    {
+        public Nest Nest { get; set; }
+
+        public DateTime LastSaveDate { get; private set; }
+
+        public string LastSavePath { get; private set; }
+
+        public Units Units { get; set; }
+
+        public void SaveAs(string path)
+        {
+            var name = Path.GetFileNameWithoutExtension(path);
+            Nest.Name = name;
+            LastSaveDate = DateTime.Now;
+            LastSavePath = path;
+
+            var writer = new NestWriter(Nest);
+            writer.Write(path);
+        }
+    }
+}