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/Quantity.cs

@@ -0,0 +1,18 @@
+namespace OpenNest
+{
+    public struct Quantity
+    {
+        public int Nested { get; internal set; }
+
+        public int Required { get; set; }
+
+        public int Remaining
+        {
+            get 
+            {
+                var x = Required - Nested;
+                return x < 0 ? 0: x; 
+            }
+        }
+    }
+}