Typst writer: propagate `lang` attribute on Span elements

The Typst writer ignores the `lang` attribute on bracketed Span elements. This means PDFs produced via the Typst writer can't meet WCAG 2.2 Success Criterion 3.1.2 (Language of Parts) for inline language changes. The HTML and LaTeX writers handle the same input appropriately. **Related.** #10965 added support for `lang` on Divs in the Typst writer; this issue covers the analogous Span case. ## Example Given `test.md`: ```markdown Hello [bonjour, monde]{lang=fr} world. ``` Run: ``` pandoc -f markdown -t typst test.md ``` ## Actual ```typst Hello bonjour, monde world. ``` ## Expected ```typst Hello #text(lang: "fr")[bonjour, monde] world. ``` Typst itself propagates `#text(lang: "...")` into the PDF's tagged structure as `/Lang` marked-content attributes (verified with Typst 0.14.2), so the downstream toolchain is ready to receive the right output. ## Other formats For comparison ### HTML ``` pandoc -f markdown -t html test.md ``` ```html <p>Hello <span lang="fr">bonjour, monde</span> world.</p> ``` ### LaTeX ``` pandoc -f markdown -t latex test.md ``` ```latex Hello \foreignlanguage{french}{bonjour, monde} world. ``` **Pandoc version**: pandoc 3.9.0.2 on macOS 26.5