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/Geometry/DefinedShape.cs

@@ -0,0 +1,42 @@
+using System.Collections.Generic;
+
+namespace OpenNest.Geometry
+{
+    public class DefinedShape
+    {
+        public DefinedShape(Shape shape)
+        {
+            Update(shape.Entities);
+        }
+
+        public DefinedShape(List<Entity> entities)
+        {
+            Update(entities);
+        }
+
+        private void Update(List<Entity> entities)
+        {
+            var shapes = Helper.GetShapes(entities);
+
+            Perimeter = shapes[0];
+            Cutouts = new List<Shape>();
+
+            for (int i = 1; i < shapes.Count; i++)
+            {
+                if (shapes[i].Left < Perimeter.Left)
+                {
+                    Cutouts.Add(Perimeter);
+                    Perimeter = shapes[i];
+                }
+                else
+                {
+                    Cutouts.Add(shapes[i]);
+                }
+            }
+        }
+
+        public Shape Perimeter { get; set; }
+
+        public List<Shape> Cutouts { get; set; }
+    }
+}