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/Geometry/GeometrySimplifierTests.cs

@@ -138,8 +138,7 @@ public class GeometrySimplifierTests
         if (!File.Exists(path))
             return; // skip if file not available
 
-        var importer = new DxfImporter();
-        var result = importer.Import(path);
+        var result = Dxf.Import(path);
         var shapes = ShapeBuilder.GetShapes(result.Entities);
 
         var simplifier = new GeometrySimplifier { Tolerance = 0.004 };