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/Spacing.cs

@@ -0,0 +1,27 @@
+namespace OpenNest
+{
+    public struct Spacing
+    {
+        public Spacing(double topBottom, double leftRight)
+        {
+            Top = Bottom = topBottom;
+            Left = Right = leftRight;
+        }
+
+        public Spacing(double left, double bottom, double right, double top)
+        {
+            Left = left;
+            Bottom = bottom;
+            Right = right;
+            Top = top;
+        }
+
+        public double Left;
+
+        public double Bottom;
+
+        public double Right;
+
+        public double Top;
+    }
+}