feat(naming): update DXF filename format to include drawing number and PT##

- Use prefix as drawing number and format as {DrawingNo} PT{ItemNo}

- Default to PT{ItemNo} when no prefix provided

ExportDXF/Services/PartExporter.cs

@@ -283,7 +283,11 @@ namespace ExportDXF.Services
                 return prefix + item.PartName;
             }
 
-            return prefix + item.ItemNo.PadLeft(2, '0');
+            var num = item.ItemNo.PadLeft(2, '0');
+            // Expected format: {DrawingNo} PT{ItemNo}
+            return string.IsNullOrWhiteSpace(prefix)
+                ? $"PT{num}"
+                : $"{prefix} PT{num}";
         }
 
         private void LogExportFailure(Item item, ExportContext context)