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/Forms/BomImportForm.cs

@@ -382,7 +382,6 @@ namespace OpenNest.Forms
             }
 
             var jobName = txtJobName.Text.Trim();
-            var importer = new DxfImporter();
             var nestsCreated = 0;
             var importErrors = new List<string>();
 
@@ -416,7 +415,7 @@ namespace OpenNest.Forms
 
                     try
                     {
-                        var result = importer.Import(part.DxfPath);
+                        var result = Dxf.Import(part.DxfPath);
 
                         var drawingName = Path.GetFileNameWithoutExtension(part.DxfPath);
                         var drawing = new Drawing(drawingName);

OpenNest/Forms/CadConverterForm.cs

@@ -74,8 +74,7 @@ namespace OpenNest.Forms
         {
             try
             {
-                var importer = new DxfImporter();
-                var result = importer.Import(file);
+                var result = Dxf.Import(file);
 
                 if (result.Entities.Count == 0)
                     return;
@@ -383,8 +382,7 @@ namespace OpenNest.Forms
                 newItems.Add(splitPath);
 
                 // Re-import geometry but keep bends from the split drawing
-                var importer = new DxfImporter();
-                var result = importer.Import(splitPath);
+                var result = Dxf.Import(splitPath);
 
                 var splitItem = new FileListItem
                 {

OpenNest/Forms/EditNestForm.cs

@@ -362,9 +362,8 @@ namespace OpenNest.Forms
             {
                 if (dlg.FilterIndex == 1)
                 {
-                    var exporter = new DxfExporter();
-                    var success = exporter.ExportPlate(PlateView.Plate, dlg.FileName);
-                    return success;
+                    Dxf.ExportPlate(PlateView.Plate, dlg.FileName);
+                    return true;
                 }
                 else if (dlg.FilterIndex == 2)
                 {
@@ -540,8 +539,7 @@ namespace OpenNest.Forms
             var plate = PlateView.Plate;
             var name = string.Format("{0}-P{1}.dxf", Nest.Name, PlateManager.CurrentIndex + 1);
             var path = Path.Combine(Path.GetTempPath(), name);
-            var exporter = new DxfExporter();
-            exporter.ExportPlate(plate, path);
+            Dxf.ExportPlate(plate, path);
 
             Process.Start(path);
         }