Generate accessible tagged PDF (PDF/UA-2) from dtbook-to-latex

## Background

The `dtbook-to-latex` script (introduced in PR #137) currently generates LaTeX that does not produce accessible tagged PDF output. This is noted in the script documentation.

Producing accessible tagged PDF (PDF/UA-2) requires:
- TeX Live 2025 or later
- LuaLaTeX (XeLaTeX is not supported for tagged PDF)
- Changes to the generated LaTeX preamble — at minimum adding `\DocumentMetadata{tagging=on}` before `\documentclass`

## References

- [LaTeX Tagging Project](https://latex3.github.io/tagging-project/) — the upstream effort adding tagged PDF support to LaTeX
- [Creating accessible PDFs in LaTeX](https://docs.overleaf.com/writing-and-editing/creating-accessible-pdfs) — practical guide on Overleaf

## What needs to be done

Investigate what changes to the generated LaTeX preamble are required and update the XSL stylesheet accordingly. The feature should probably be opt-in (a new script option) given the TeX Live 2025 requirement, which many users may not have.

Start with the dtbook-to-latex script documentation and its XSL stylesheet, then inspect how the generated LaTeX preamble and script options are assembled. Test the output with LuaLaTeX on TeX Live 2025 or later; done means the selected mode produces accessible tagged PDF output while preserving a usable path for users without that TeX Live version.

Written by the indexing model from the issue text.