CI: oup_v1 render fails with '! Undefined control sequence \authormark' after upstream CTAN update

[cderv]

Symptom

R-CMD-check is failing across nearly the entire matrix (16/18 jobs) on a fresh
install of TinyTeX. The failure is the same on every OS and pandoc combination
that exercises the oup_v1 test:

Rendering the oup_v1 format...
[1/1] install: oup-authoring-template [27k]
...
! Undefined control sequence.
l.241 \authormark
                 {Alice Anonymous et al.}

Error: LaTeX failed to compile oup_v1_article.tex.

The two jobs that pass (ubuntu-latest (release) [Pandoc 2.5] and [Pandoc 2.7.3]) only pass because test_format("oup_v1", skip = !rmarkdown::pandoc_available("2.10")) in tests/testit/test-formats.R:70
skips OUP v1 on those older pandoc versions.

Example failing run: https://github.com/rstudio/rticles/actions/runs/25806163370

Cause

Independent of any rticles change. The upstream
oup-authoring-template class
on CTAN has been updated and no longer defines \authormark. TinyTeX
auto-installs the latest version from CTAN at test time, so every CI run
freshly downloads the broken-for-our-use version.

Our template still emits \authormark{...} from
inst/rmarkdown/templates/oup_v1/resources/template.tex:384-388.

R-CMD-check on main was green as recently as #599 (Update to JSS 3.6), so
the regression entered via an upstream CTAN release, not via an rticles
commit. PR #601 (Copernicus 7.14) is the first PR to hit it; its Copernicus
changes are unrelated and the Copernicus rendering itself passes in every
job before oup_v1 fails.

Suggested fixes

Three options, roughly in increasing effort:

1. Pin the class file: vendor oup-authoring-template.cls (and any
   companions) into inst/rmarkdown/templates/oup_v1/skeleton/ at the last
   working version so it shadows CTAN — same pattern we already use for
   copernicus.cls, agujournal2018.cls, etc.
2. Patch the template: replace \authormark with whatever the new
   upstream class expects (need to read the new .dtx to know). If only the
   command name changed, this is a one-line fix in
   inst/rmarkdown/templates/oup_v1/resources/template.tex:384-388.
3. Defensive macro: wrap with
   \providecommand{\authormark}[1]{} or
   \@ifundefined{authormark}{\let\authormark\@gobble}{} so missing macros
   degrade silently. Cheapest, but loses the running-head behavior.

Tracking #409 is also relevant — it already notes the OUP template needs an
update; this is now blocking CI.

Workaround for incoming PRs

Until this lands, contributors can ignore the red R-CMD-check if the only
failing test is test-formats.R -> oup_v1. The rest of the test suite is
unaffected.