refactor: extract OpenNest.IO class library from WinForms project

Move DxfImporter, DxfExporter, NestReader, NestWriter, ProgramReader,
and Extensions into a new OpenNest.IO class library. The WinForms project
now references OpenNest.IO instead of ACadSharp directly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.IO/DxfImporter.cs

@@ -0,0 +1,117 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using ACadSharp;
+using ACadSharp.IO;
+using OpenNest.Geometry;
+
+namespace OpenNest.IO
+{
+    public class DxfImporter
+    {
+        public int SplinePrecision { get; set; }
+
+        public DxfImporter()
+        {
+        }
+
+        private List<Entity> GetGeometry(CadDocument doc)
+        {
+            var entities = new List<Entity>();
+            var lines = new List<Line>();
+            var arcs = new List<Arc>();
+
+            foreach (var entity in doc.Entities)
+            {
+                switch (entity)
+                {
+                    case ACadSharp.Entities.Line line:
+                        lines.Add(line.ToOpenNest());
+                        break;
+
+                    case ACadSharp.Entities.Arc arc:
+                        arcs.Add(arc.ToOpenNest());
+                        break;
+
+                    case ACadSharp.Entities.Circle circle:
+                        entities.Add(circle.ToOpenNest());
+                        break;
+
+                    case ACadSharp.Entities.Spline spline:
+                        lines.AddRange(spline.ToOpenNest());
+                        break;
+
+                    case ACadSharp.Entities.LwPolyline lwPolyline:
+                        lines.AddRange(lwPolyline.ToOpenNest());
+                        break;
+
+                    case ACadSharp.Entities.Polyline polyline:
+                        lines.AddRange(polyline.ToOpenNest());
+                        break;
+
+                    case ACadSharp.Entities.Ellipse ellipse:
+                        lines.AddRange(ellipse.ToOpenNest(SplinePrecision));
+                        break;
+                }
+            }
+
+            Helper.Optimize(lines);
+            Helper.Optimize(arcs);
+
+            entities.AddRange(lines);
+            entities.AddRange(arcs);
+
+            return entities;
+        }
+
+        public bool GetGeometry(Stream stream, out List<Entity> geometry)
+        {
+            var success = false;
+
+            try
+            {
+                using (var reader = new DxfReader(stream))
+                {
+                    var doc = reader.Read();
+                    geometry = GetGeometry(doc);
+                    success = true;
+                }
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine(ex.Message);
+                geometry = new List<Entity>();
+            }
+            finally
+            {
+                if (stream != null)
+                    stream.Close();
+            }
+
+            return success;
+        }
+
+        public bool GetGeometry(string path, out List<Entity> geometry)
+        {
+            var success = false;
+
+            try
+            {
+                using (var reader = new DxfReader(path))
+                {
+                    var doc = reader.Read();
+                    geometry = GetGeometry(doc);
+                    success = true;
+                }
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine(ex.Message);
+                geometry = new List<Entity>();
+            }
+
+            return success;
+        }
+    }
+}