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/NestItem.cs

@@ -0,0 +1,35 @@
+namespace OpenNest
+{
+    public class NestItem
+    {
+        /// <summary>
+        /// The drawing to be nested.
+        /// </summary>
+        public Drawing Drawing { get; set; }
+
+        /// <summary>
+        /// Priority of the part determines nesting order. Highest priority will be nested first.
+        /// </summary>
+        public int Priority { get; set; }
+
+        /// <summary>
+        /// The number of parts to be nested.
+        /// </summary>
+        public int Quantity { get; set; }
+
+        /// <summary>
+        /// The rotation step in radians.
+        /// </summary>
+        public double StepAngle { get; set; }
+
+        /// <summary>
+        /// The rotation start angle in radians.
+        /// </summary>
+        public double RotationStart { get; set; }
+
+        /// <summary>
+        /// The rotation end angle in radians.
+        /// </summary>
+        public double RotationEnd { get; set; }
+    }
+}