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/Splitting/SplitDxfWriterEtchLayerTests.cs

@@ -185,8 +185,7 @@ public class SplitDxfWriterEtchLayerTests
                 writer.Write(tempPath, splitDrawing);
 
                 // Re-import via DxfImporter (same path as CadConverterForm)
-                var importer = new DxfImporter();
-                var result = importer.Import(tempPath);
+                var result = Dxf.Import(tempPath);
 
                 // ETCH entities should be filtered during import (like BEND)
                 var etchEntities = result.Entities