feat: add DxfImporter.Import returning entities + CadDocument

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

OpenNest.IO/DxfImportResult.cs

@@ -0,0 +1,12 @@
+using ACadSharp;
+using OpenNest.Geometry;
+using System.Collections.Generic;
+
+namespace OpenNest.IO
+{
+    public class DxfImportResult
+    {
+        public List<Entity> Entities { get; set; } = new();
+        public CadDocument Document { get; set; }
+    }
+}