fix: propagate fig-alt to Typst image alt text for PDF/UA-1

[gordonwoodhull]

Summary

• Fix fig-alt not being propagated to Typst's image(alt: "...") parameter for computed figures (R/Python code chunks)
• Refactor figures.lua to make fig-alt propagation format-agnostic instead of having separate HTML/LaTeX/Typst branches
• Add test case verifying fig-alt on R code chunks produces image(alt: ...) in Typst output

Details

The fig-alt propagation in quarto-pre/figures.lua only handled HTML and LaTeX outputs. The Typst post-filter in quarto-post/typst.lua correctly looks for image.attributes["alt"] and generates image(alt: "..."), but the attribute was never set for Typst output.

The fix refactors the three format-specific branches into a single block:

• HTML: sets alt on the float itself (unchanged behavior)
• All other formats (LaTeX, Typst, etc.): walks into float.content to set alt on Image nodes

This means any future format will also automatically get fig-alt propagation.

Closes #14108

Test plan

• New test typst-fig-alt-propagation.qmd passes (verified fig-alt from R chunk → image(alt: ...) in .typ)
• Existing typst-image-alt-text.qmd passes (caption-as-alt, explicit alt)
• Existing equations-alt.qmd passes (html, pdf, typst)
• Existing ua-image-alt-text.qmd passes (pdf, typst)

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