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/DrawingSplitterTests.cs

@@ -369,8 +369,7 @@ public class DrawingSplitterTests
             var writer = new OpenNest.IO.SplitDxfWriter();
             writer.Write(tempPath, results[0]);
 
-            var reimporter = new OpenNest.IO.DxfImporter();
-            var reimportResult = reimporter.Import(tempPath);
+            var reimportResult = OpenNest.IO.Dxf.Import(tempPath);
 
             var afterArcs = reimportResult.Entities.OfType<Arc>().Count();
             var afterCircles = reimportResult.Entities.OfType<Circle>().Count();