feat: add chunk option fig.note for figure notes (#2022)

[yihui]

Closes #2022.

Adds a new chunk option fig.note to place a note (e.g., a source or explanatory note) below a figure, separate from its caption. Long-requested feature (10+ comments on the issue).

Behavior by output format

• LaTeX/PDF: emits \figurenote{...} inside the figure environment, after \caption. A default \providecommand{\figurenote} (footnotesize italic) is shipped, so it works out of the box on both the .Rnw and .Rmd→pandoc paths. Users can override it in the preamble, e.g. with the floatrow package:

  \newcommand{\figurenote}[1]{\floatfoot{#1}}

• HTML: places the note in <p class="figure-note"> inside the figure <div>. A note with no caption still gets a figure <div> (and no empty caption paragraph). A default .figure-note style is added to inst/misc/vignette.css.
• Typst: emits an emphasized small block after #figure.
• Office formats (docx/pptx/…) have no semantic equivalent and drop the note silently, consistent with fig.alt.

Plumbing

• fig.note added to eval.after (inline R in the note evaluated after the chunk), to .recyle.opts (recycled across multiple plots per chunk), and to need_special_plot_hook() so .Rmd→LaTeX routes through hook_plot_tex().

Example

```{r, fig.cap="Monthly sales", fig.note="Source: internal data."}
plot(1:10)
```

Tests

Added cases to tests/testit/test-hooks-latex.R (note placement + empty/NA note) and tests/testit/test-hooks-md.R (HTML figure-note paragraph, with and without a caption). Existing plot/hook tests pass.

🤖 Generated with Claude Code