fix: enable CreateDefaults on DxfReader to fix missing ACAD_GROUP dictionary

SolidWorks DXF exports don't include the ACAD_GROUP entry in the Named
Object Dictionary. Without it, AutoCAD reports "GroupTable dictionary was
not defined in NamedObject dictionary" when opening the file.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

EtchBendLines/Etcher.cs

@@ -34,6 +34,7 @@ namespace EtchBendLines
             try
             {
                 using var reader = new DxfReader(path);
+                reader.Configuration.CreateDefaults = true;
                 return reader.Read()
                     ?? throw new InvalidOperationException("DXF load returned null");
             }