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/Api/NestRunnerTests.cs

@@ -70,8 +70,7 @@ public class NestRunnerTests
         var pgm = ConvertGeometry.ToProgram(shape);
         var path = Path.Combine(Path.GetTempPath(), $"test-{Guid.NewGuid()}.dxf");
 
-        var exporter = new DxfExporter();
-        exporter.ExportProgram(pgm, path);
+        Dxf.ExportProgram(pgm, path);
 
         return path;
     }