feat: serialize subscript and superscript in Markdown export

[LucasArray]

Summary

The Markdown serializer drops subscript and superscript formatting. A text span marked Formatting(script=Script.SUB) or Script.SUPER is written out as plain text, so a subscript like the 2 in H2O or an exponent like E=mc2 becomes an ordinary digit with no way to recover it. The HTML serializer already preserves these as <sub>/<sup>; the Markdown serializer never implemented the hooks.

This adds them. #319 introduced the sub/superscript model and the HTML support but intentionally left Markdown out, since the Pandoc ~x~/^x^ syntax is uncommon and does not render in editors like VS Code. Emitting inline <sub>/<sup> avoids that: it renders on GitHub and in any CommonMark viewer, and it matches what the HTML serializer already produces, so Markdown and HTML stay in sync.

Addresses docling-project/docling#520.

Changes

• docling_core/transforms/serializer/markdown.py: implement serialize_subscript and serialize_superscript, returning <sub>{text}</sub> and <sup>{text}</sup>. They run in post_process after the content is HTML-escaped, like the existing bold/italic/strikethrough hooks, so only the wrapping tags are literal.
• docling_core/transforms/serializer/plain_text.py: PlainTextDocSerializer subclasses the Markdown serializer, so it would otherwise inherit the new tags. Override both hooks to return the text unchanged, consistent with how it already strips bold/italic/strikethrough.
• test/test_serialization.py: add test_md_subscript_formatting and test_md_superscript_formatting.
• Regenerated the Markdown reference data under test/data/doc/. The shared test fixture already contains sub/superscript spans, so their Markdown ground truth now renders <sub>/<sup> (one changed line per file). Note: this updates reference test data, which requires a double review per CONTRIBUTING.

Example

doc.add_text(label=DocItemLabel.TEXT, text="H2O", formatting=Formatting(script=Script.SUB))
doc.export_to_markdown()

Before: H2O
After: <sub>H2O</sub>

I also checked this directly by exporting the same document with and without the change and rendering both outputs. The subscript and superscript text now displays as actual sub/superscript instead of flat text.

Testing

All run locally and passing:

• uv run pytest: 544 passed, 6 skipped
• uv run ruff check and uv run ruff format --check: clean
• uv run mypy docling_core test: clean
• New unit tests assert <sub>/<sup> in the Markdown output; the plain-text serializer test confirms the tags are stripped there

[github-actions]

✅ DCO Check Passed

Thanks @LucasArray, all your commits are properly signed off. 🎉

[mergify]

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