Record the rule that Python 3D figures use plotly in HTML, with matplotlib for static output #496

Ezra decided (2026-11-05) that Python 3D figures use plotly in HTML and slide output, with a matplotlib figure for PDF and Word.
The 3D-graphs guidance in /home/user/psw/chapters/slides-posters-figures.qmd (lines 104-105 and 213-217) names plotly only through its R documentation (https://plotly.com/r/) and does not say what a Python tab does, so the Python tabs in the notes repos still draw 3D surfaces with matplotlib.

Proposed change to that chapter:

• Say that a Python tab of a 3D figure uses plotly for HTML and slides, and links the plotly Python documentation.
• Say that the PDF and Word output keep a matplotlib figure at a fixed angle, so no static-export dependency (kaleido) is needed.
• Record the build details the pilot found: emit fig.to_html(full_html=False, include_plotlyjs="cdn") inside a {=html} fence from one print under results: asis (fig.show() needs ipython, and several prints split knitr's output); end figure-returning calls with ; so reticulate does not auto-print them; add plotly to the page's reticulate::py_require.

The pilot is Morrison-Lab/mds#660 (/home/user/mds/_subfiles/vector-calc/_fig-second-order-condition-3d.qmd, issue Morrison-Lab/mds#659).
This issue waits for that PR to render well before the rule is written, and the other Python 3D figures follow in later PRs.

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Agent-authored: posted by Claude Code on behalf of d-morrison.