Fix Pandoc subdocument XML namespace handling

[aka-debug-jie]

Summary

• serialize each direct subdocument body child so inherited XML namespaces remain in scope
• preserve the existing sectPr removal behavior
• add a Pandoc-generated DOCX regression fixture with an embedded image

Validation

• reproduced the original Namespace prefix a on graphic is not defined failure on the unfixed baseline
• verified the rendered DOCX opens with python-docx and its word/document.xml parses with lxml
• verified the merged document retains one inline image and namespace-resolved DrawingML/PictureML elements
• passed the complete 37-script test suite, related subdocument/header-footer tests, flake8, package builds, and archive audits

Closes #620