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/Drawing.cs

@@ -0,0 +1,117 @@
+using System.Drawing;
+using System.Linq;
+using OpenNest.CNC;
+
+namespace OpenNest
+{
+    public class Drawing
+    {
+        private Program program;
+
+        public Drawing()
+            : this(string.Empty, new Program())
+        {
+        }
+
+        public Drawing(string name)
+            : this(name, new Program())
+        {
+        }
+
+        public Drawing(string name, Program pgm)
+        {
+            Name = name;
+            Material = new Material();
+            Program = pgm;
+            Constraints = new NestConstraints();
+            Source = new SourceInfo();
+        }
+
+        public string Name { get; set; }
+
+        public string Customer { get; set; }
+
+        public int Priority { get; set; }
+
+        public Quantity Quantity;
+
+        public Material Material { get; set; }
+
+        public Program Program
+        {
+            get { return program; }
+            set
+            {
+                program = value;
+                UpdateArea();
+            }
+        }
+
+        public Color Color { get; set; }
+
+        public NestConstraints Constraints { get; set; }
+
+        public SourceInfo Source { get; set; }
+
+        public double Area { get; protected set; }
+
+        public void UpdateArea()
+        {
+            var geometry = ConvertProgram.ToGeometry(Program).Where(entity => entity.Layer != SpecialLayers.Rapid);
+            var shapes = Helper.GetShapes(geometry);
+
+            if (shapes.Count == 0)
+                return;
+
+            var areas = new double[shapes.Count];
+
+            for (int i = 0; i < shapes.Count; i++)
+            {
+                var shape = shapes[i];
+                areas[i] = shape.Area();
+            }
+
+            int largestAreaIndex = 0;
+
+            for (int i = 1; i < areas.Length; i++)
+            {
+                var area = areas[i];
+
+                if (area > areas[largestAreaIndex])
+                    largestAreaIndex = i;
+            }
+
+            var outerArea = areas[largestAreaIndex];
+
+            Area = outerArea - (areas.Sum() - outerArea);
+        }
+
+        public override bool Equals(object obj)
+        {
+            if (obj is Drawing == false)
+                return false;
+
+            var dwg = (Drawing)obj;
+
+            return Name == dwg.Name;
+        }
+
+        public override int GetHashCode()
+        {
+            return Name.GetHashCode();
+        }
+    }
+
+    public class SourceInfo
+    {
+        /// <summary>
+        /// Path to the source file.
+        /// </summary>
+        public string Path { get; set; }
+
+        /// <summary>
+        /// Offset distances to the original location.
+        /// </summary>
+        public Vector Offset { get; set; }
+    }
+}