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/Controls/NumericUpDown.cs

@@ -0,0 +1,40 @@
+using System;
+
+namespace OpenNest.Controls
+{
+    public class NumericUpDown : System.Windows.Forms.NumericUpDown
+    {
+        private string suffix;
+
+        
+
+        public NumericUpDown()
+        {
+            suffix = string.Empty;
+        }
+
+        public string Suffix
+        {
+            get { return suffix; }
+            set 
+            { 
+                suffix = value;
+                UpdateEditText();
+            }
+        }
+
+        protected override void OnEnter(EventArgs e)
+        {
+            base.OnEnter(e);
+            this.Select(0, Text.Length);
+        }
+
+        protected override void UpdateEditText()
+        {
+            if (Suffix != null)
+                Text = Value.ToString("N" + DecimalPlaces) + Suffix;
+            else
+                base.UpdateEditText();
+        }
+    }
+}