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/EvenOdd.cs

@@ -0,0 +1,15 @@
+namespace OpenNest
+{
+    public static class EvenOdd
+    {
+        public static bool IsEven(this int i)
+        {
+            return (i % 2) == 0;
+        }
+
+        public static bool IsOdd(this int i)
+        {
+            return (i % 2) == 1;
+        }
+    }
+}