refactor: merge DxfImporter and DxfExporter into single static Dxf class

Consolidated two stateless classes into one unified API: Dxf.Import(),
Dxf.GetGeometry(), Dxf.ExportPlate(), Dxf.ExportProgram(). Export
state moved into a private ExportContext. Removed bool+out pattern
from GetGeometry in favor of returning empty list on failure.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Tests/BestFit/BestFitOverlapTests.cs

@@ -32,8 +32,7 @@ public class BestFitOverlapTests
         if (!File.Exists(DxfPath))
             return null;
 
-        var importer = new DxfImporter();
-        importer.GetGeometry(DxfPath, out var geometry);
+        var geometry = Dxf.GetGeometry(DxfPath);
         var pgm = ConvertGeometry.ToProgram(geometry);
         return new Drawing("PT16", pgm);
     }