Replace fig-pos [H] with [h] when PDF tagging is active

[gordonwoodhull]

Summary

• Fix broken PDF tag structure for figures when pdf-standard: ua-2 is set
• Replace [H] float placement with [h] when PDF tagging is active, across all filter pipeline locations
• Also fixes the previously-known non-cross-referenceable figure caption issue

Details

The [H] specifier (from LaTeX's float package) redefines the figure environment in a way that breaks lualatex's PDF tagging system — /Caption and /Figure end up as direct children of /Document instead of being properly nested. This causes verapdf to report <Document> shall not contain <Caption>.

Multiple code paths inject [H]: knitr/Jupyter execution engines (for echoed code chunks), pandoc3_figure.lua (for non-cross-referenceable figures), and quarto-post/latex.lua (for panel layouts). Cross-referenceable static images were unaffected only because they happen not to get [H] — the LaTeX output is structurally identical in all cases.

Empirical testing of placement specifiers with pdf-standard: ua-2:

Specifier	verapdf result
(none)	PASSED
[h]	PASSED
[htbp]	PASSED
[H]	FAILED — <Document> shall not contain <Caption>

Only [H] (from the float package) breaks tagging. All standard LaTeX specifiers work correctly. [h] is the closest semantic equivalent to [H] ("place here") without the float package's incompatible reimplementation.

Upstream: latex3/tagging-project#890

Changes

• `src/format/pdf/format-pdf.ts` — expose `pdf-tagging` metadata to Lua filters
• `src/resources/filters/quarto-pre/figures.lua` — replace `fig-pos='H'` with `'h'` when tagging is active
• `src/resources/filters/quarto-post/latex.lua` — same for panel layouts and star environments
• `src/resources/filters/layout/pandoc3_figure.lua` — same for non-cross-referenceable figures
• `tests/docs/smoke-all/pdf-standard/ua-code-chunk-figure.qmd` — new test for R code chunk figure with ua-2
• `tests/docs/smoke-all/pdf-standard/ua2-unlabeled-figure-caption.qmd` — updated from known-failure to passing

Test plan

• New test `ua-code-chunk-figure.qmd` passes (R code chunk + ua-2 + verapdf)
• Updated `ua2-unlabeled-figure-caption.qmd` now passes
• Full `pdf-standard/` test suite: 40 passed, 0 failed
• TypeScript typecheck passes

Fixes #14164

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[gordonwoodhull]

This is apparently not the way - there are hooks that can be set in order to make the float package work.

EDIT: Further research shows that no, there are no such hooks for float. But I don’t think making our floats into non-floats will be an acceptable solution.