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.Engine/CirclePacking/Bin.cs

@@ -0,0 +1,30 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace OpenNest.CirclePacking
+{
+    internal class Bin : Box
+    {
+        public Bin()
+        {
+            Items = new List<Item>();
+        }
+
+        public List<Item> Items { get; set; }
+
+        public double Density()
+        {
+            return Items.Sum(i => i.Area()) / Area();
+        }
+
+        public object Clone()
+        {
+            return new Bin
+            {
+                Location = this.Location,
+                Size = this.Size,
+                Items = new List<Item>(Items)
+            };
+        }
+    }
+}