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/IO/Extensions.cs

@@ -0,0 +1,164 @@
+using System.Collections.Generic;
+using System.Linq;
+using OpenNest.Geometry;
+
+namespace OpenNest.IO
+{
+    internal static class Extensions
+    {
+        public static Vector ToOpenNest(this netDxf.Vector2 v)
+        {
+            return new Vector(v.X, v.Y);
+        }
+
+        public static Vector ToOpenNest(this netDxf.Vector3 v)
+        {
+            return new Vector(v.X, v.Y);
+        }
+
+        public static Vector ToOpenNest(this netDxf.Entities.Polyline2DVertex v)
+        {
+            return new Vector(v.Position.X, v.Position.Y);
+        }
+
+        public static Arc ToOpenNest(this netDxf.Entities.Arc arc)
+        {
+            return new Arc(
+                arc.Center.X, arc.Center.Y, arc.Radius,
+                Angle.ToRadians(arc.StartAngle),
+                Angle.ToRadians(arc.EndAngle))
+                {
+                    Layer = arc.Layer.ToOpenNest()
+                };
+        }
+
+        public static Circle ToOpenNest(this netDxf.Entities.Circle circle)
+        {
+            return new Circle(
+                circle.Center.X, circle.Center.Y,
+                circle.Radius)
+                {
+                    Layer = circle.Layer.ToOpenNest()
+                };
+        }
+
+        public static Line ToOpenNest(this netDxf.Entities.Line line)
+        {
+            return new Line(
+                line.StartPoint.X, line.StartPoint.Y,
+                line.EndPoint.X, line.EndPoint.Y)
+                {
+                    Layer = line.Layer.ToOpenNest()
+                };
+        }
+
+        public static List<Line> ToOpenNest(this netDxf.Entities.Spline spline, int precision = 200)
+        {
+            var lines = new List<Line>();
+            var pts = spline.PolygonalVertexes(precision);
+
+            if (pts.Count == 0)
+                return lines;
+
+            var lastPoint = pts[0].ToOpenNest();
+
+            for (int i = 1; i < pts.Count; i++)
+            {
+                var nextPoint = pts[i].ToOpenNest();
+
+                lines.Add(new Line(
+                    lastPoint,
+                    nextPoint) { Layer = spline.Layer.ToOpenNest() });
+
+                lastPoint = nextPoint;
+            }
+
+            if (spline.IsClosed)
+                lines.Add(new Line(lastPoint, pts[0].ToOpenNest()) { Layer = spline.Layer.ToOpenNest() });
+
+            return lines;
+        }
+
+        public static List<Line> ToOpenNest(this netDxf.Entities.Polyline3D polyline)
+        {
+            var lines = new List<Line>();
+
+            if (polyline.Vertexes.Count == 0)
+                return lines;
+
+            var lastPoint = polyline.Vertexes[0].ToOpenNest();
+
+            for (int i = 1; i < polyline.Vertexes.Count; i++)
+            {
+                var nextPoint = polyline.Vertexes[i].ToOpenNest();
+
+                lines.Add(new Line(
+                    lastPoint,
+                    nextPoint) { Layer = polyline.Layer.ToOpenNest() });
+
+                lastPoint = nextPoint;
+            }
+
+            if (polyline.IsClosed)
+                lines.Add(new Line(lastPoint, polyline.Vertexes[0].ToOpenNest()) { Layer = polyline.Layer.ToOpenNest() });
+
+            return lines;
+        }
+
+        public static List<Line> ToOpenNest(this netDxf.Entities.Polyline2D polyline)
+        {
+            var lines = new List<Line>();
+
+            if (polyline.Vertexes.Count == 0)
+                return lines;
+
+            var lastPoint = polyline.Vertexes[0].ToOpenNest();
+
+            for (int i = 1; i < polyline.Vertexes.Count; i++)
+            {
+                var nextPoint = polyline.Vertexes[i].ToOpenNest();
+
+                lines.Add(new Line(
+                    lastPoint,
+                    nextPoint) { Layer = polyline.Layer.ToOpenNest() });
+
+                lastPoint = nextPoint;
+            }
+
+            if (polyline.IsClosed)
+                lines.Add(new Line(lastPoint, polyline.Vertexes[0].ToOpenNest()) { Layer = polyline.Layer.ToOpenNest() });
+
+            return lines;
+        }
+
+        public static List<Line> ToOpenNest(this netDxf.Entities.Ellipse ellipse, int precision = 200)
+        {
+            var lines = ellipse.ToPolyline2D(precision).ToOpenNest();
+
+            if (lines.Count < 2)
+                return lines;
+
+            var first = lines.First();
+            var last = lines.Last();
+
+            // workaround for ellipse.ToPolyline not connecting the last and first point.
+            lines.Add(new Line(last.EndPoint, first.StartPoint) { Layer = ellipse.Layer.ToOpenNest() });
+
+            return lines;
+        }
+
+        public static Layer ToOpenNest(this netDxf.Tables.Layer layer)
+        {
+            return new Layer(layer.Name)
+            {
+                Color = layer.Color.ToColor(),
+                IsVisible = layer.IsVisible
+            };
+        }
+
+        public static netDxf.Vector2 ToNetDxf(this Vector v)
+        {
+            return new netDxf.Vector2(v.X, v.Y);
+        }
+    }
+}