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/Size.cs

@@ -0,0 +1,55 @@
+using System;
+
+namespace OpenNest
+{
+    public struct Size
+    {
+        public Size(double width, double height)
+        {
+            Height = height;
+            Width = width;
+        }
+
+        public double Height;
+
+        public double Width;
+
+        public static Size Parse(string size)
+        {
+            var a = size.ToUpper().Split('X');
+
+            if (a.Length > 2)
+                throw new FormatException("Invalid size format.");
+
+            var height = double.Parse(a[0]);
+            var width = double.Parse(a[1]);
+
+            return new Size(width, height);
+        }
+
+        public static bool TryParse(string s, out Size size)
+        {
+            try
+            {
+                size = Parse(s);
+            }
+            catch
+            {
+                size = new Size(0, 0);
+                return false;
+            }
+
+            return true;
+        }
+
+        public override string ToString()
+        {
+            return string.Format("{0} x {1}", Height, Width);
+        }
+
+        public string ToString(int decimalPlaces)
+        {
+            return string.Format("{0} x {1}", Math.Round(Height, decimalPlaces), Math.Round(Width, decimalPlaces));
+        }
+    }
+}