Fix natbib citations rendering inline instead of in the margin

[cderv]

When tufte::tufte_handout() (or tufte_book()) is used with citation_package: natbib, citations render inline as plain author-year text with a bibliography only at the bottom, instead of appearing as margin sidenotes like tufte-latex does for hand-written \cite{} calls.

Root Cause

Pandoc with citation_package: natbib emits \citep{key} for [@key] and \citet{key} for @key, never the bare \cite{} form. The tufte-latex classes only redefine \cite to route through the margin sidenote machinery (inst/rmarkdown/templates/tufte_handout/patches/tufte-common.def, line 970); natbib's own \citep and \citet are left untouched, so they render inline like in any non-tufte document.

Fix

Patch tufte-common.def to redefine \citep and \citet via \RenewDocumentCommand, guarded by the existing @tufte@loadnatbib boolean:

\ifthenelse{\boolean{@tufte@loadnatbib}}{%
  \RenewDocumentCommand{\citep}{o o m}{\cite{#3}}%
  \RenewDocumentCommand{\citet}{o o m}{\citeauthor{#3}~(\citeyear{#3})\cite{#3}}%
}{}

\citep now produces a margin sidenote (superscript marker in body, full reference in the margin). \citet keeps natbib's textual "Author (Year)" form inline AND adds the full reference to the margin — the proper Tufte book style, preserving prose for textual citations. The redefinitions delegate to \cite rather than \@tufte@normal@cite directly so they pick up tufte's in-sidenote swap when nested inside \sidenote{}.

\RenewDocumentCommand is available in the LaTeX kernel since 2020-10, so no extra \usepackage{xparse} is needed.

The patch is also registered in tools/update-tufte-common-def.R so future upstream refreshes re-apply it automatically.

Known limitations

• Optional \citep[prefix][locator]{key} arguments are dropped. [@key, p. 5] places the entry in the margin but does not echo "p. 5".
• \citeyearpar (emitted for [-@key] suppress-author) is intentionally left at natbib's default inline rendering.

Test plan

• Render tufte::tufte_handout() with citation_package: natbib and [@key] — superscript in body, full entry in margin
• Same with @key — body shows "Author (Year)" + superscript, margin shows full entry
• Multi-key [@a; @b] — single superscript, margin shows both entries joined with "and"
• [-@key] — renders inline as (YEAR), no margin entry (by design)
• tufte::tufte_book(), tufte_handout2(), tufte_book2() behave the same
• Citation inside ^[footnote] does not error
• devtools::test() passes

Fixes #48