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;
     }

OpenNest.Tests/Bending/SolidWorksBendDetectorTests.cs

@@ -35,8 +35,7 @@ public class SolidWorksBendDetectorTests
         var path = Path.Combine(AppContext.BaseDirectory, "Bending", "TestData", "4526 A14 PT11 Test.dxf");
         Assert.True(File.Exists(path), $"Test DXF not found: {path}");
 
-        var importer = new OpenNest.IO.DxfImporter();
-        var result = importer.Import(path);
+        var result = OpenNest.IO.Dxf.Import(path);
 
         // EllipseConverter now produces arcs directly during import,
         // so the imported entities should contain Arc instances from the ellipses
@@ -61,8 +60,7 @@ public class SolidWorksBendDetectorTests
         var path = Path.Combine(AppContext.BaseDirectory, "Bending", "TestData", "4526 A14 PT11.dxf");
         Assert.True(File.Exists(path), $"Test DXF not found: {path}");
 
-        var importer = new OpenNest.IO.DxfImporter();
-        var result = importer.Import(path);
+        var result = OpenNest.IO.Dxf.Import(path);
 
         // The DXF has 2 trimmed ellipses forming an oblong slot.
         // Trimmed ellipses must not generate a closing chord line.

OpenNest.Tests/BestFit/BestFitOverlapTests.cs

@@ -32,8 +32,7 @@ public class BestFitOverlapTests
         if (!File.Exists(DxfPath))
             return null;
 
-        var importer = new DxfImporter();
-        importer.GetGeometry(DxfPath, out var geometry);
+        var geometry = Dxf.GetGeometry(DxfPath);
         var pgm = ConvertGeometry.ToProgram(geometry);
         return new Drawing("PT16", pgm);
     }

OpenNest.Tests/Engine/EngineOverlapTests.cs

@@ -20,8 +20,7 @@ public class EngineOverlapTests
         if (!System.IO.File.Exists(DxfPath))
             return null;
 
-        var importer = new DxfImporter();
-        importer.GetGeometry(DxfPath, out var geometry);
+        var geometry = Dxf.GetGeometry(DxfPath);
         var pgm = ConvertGeometry.ToProgram(geometry);
         return new Drawing("PT15", pgm);
     }

OpenNest.Tests/Geometry/EllipseConverterTests.cs

@@ -228,8 +228,7 @@ public class EllipseConverterTests
             using (var writer = new ACadSharp.IO.DxfWriter(stream, doc, false))
                 writer.Write();
 
-            var importer = new OpenNest.IO.DxfImporter();
-            var result = importer.Import(tempPath);
+            var result = OpenNest.IO.Dxf.Import(tempPath);
 
             var arcCount = result.Entities.Count(e => e is Arc);
             var lineCount = result.Entities.Count(e => e is Line);

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 };

OpenNest.Tests/IO/DxfRoundtripTests.cs

@@ -11,17 +11,14 @@ public class DxfRoundtripTests
     private static List<Entity> ExportAndReimport(List<Entity> geometry)
     {
         var program = ConvertGeometry.ToProgram(geometry);
-        var exporter = new DxfExporter();
-        var importer = new DxfImporter();
-
         using var exportStream = new MemoryStream();
-        exporter.ExportProgram(program, exportStream);
+        Dxf.ExportProgram(program, exportStream);
         var bytes = exportStream.ToArray();
 
         var importStream = new MemoryStream(bytes);
-        var success = importer.GetGeometry(importStream, out var reimported);
+        var reimported = Dxf.GetGeometry(importStream);
 
-        Assert.True(success, "Failed to re-import exported DXF");
+        Assert.NotEmpty(reimported);
         return reimported;
     }
 

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();

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

OpenNest.Tests/Strategies/StrategyOverlapTests.cs

@@ -23,8 +23,7 @@ public class StrategyOverlapTests
         if (!System.IO.File.Exists(DxfPath))
             return null;
 
-        var importer = new DxfImporter();
-        importer.GetGeometry(DxfPath, out var geometry);
+        var geometry = Dxf.GetGeometry(DxfPath);
         var pgm = ConvertGeometry.ToProgram(geometry);
         return new Drawing("PT15", pgm);
     }