Fix oup_v1 render failure after upstream CTAN \authormark removal

[cderv]

Rendering oup_v1 (and all R-CMD-check jobs that exercise it) fails on fresh TinyTeX installs:

! Undefined control sequence.
l.241 \authormark
                 {Alice Anonymous et al.}

Root cause

The January 3 2026 CTAN release of oup-authoring-template v1.2 removed the \authormark macro definition (the cls file's internal \lastmodifieddate is 2025/11/17, but CI broke when the package was published on CTAN and TinyTeX started auto-installing the new version).

\authormark was an internal macro of the class, never part of its documented public API. The rticles oup_v1 template still emitted \authormark{...} for the running head, so every render fails on TinyTeX (which auto-installs the latest CTAN version at render time).

The documented running-head mechanism in v1.2 is the optional argument of \title (\title[short]{long}), which the template already uses (\title[$short_title$]{$title$} in template.tex:340). That routes through \short@title → \titlemark, never through the path that calls \authormark. OUP's own example file on CTAN no longer references \authormark either.

A structural diff of v1.1 → v1.2 confirms \authormark is the only macro definition removed; ~40 new macros were added (sideways figures/tables, box environments, \figalttext, \wordcount, etc.), none required by our template.

Fix

Drop the template block that emitted \authormark{...}. On TeX Live ≤ 2024 (cls v1.1), the only visible effect is that the even-page running head changes from "Author et al." to the short title — which is what OUP's v1.2 docs now prescribe on both pages anyway.

The YAML authormark key was undocumented but accepted by the Pandoc template before this change, so users who discovered it would silently lose the value after the upgrade. Add a pre_knit hook — mirroring the pattern in ams_article(), tf_article(), and the springer warning in R/article.R — that warns at render time when YAML still sets authormark.

Verification

End-to-end local render with the patched template against a freshly auto-installed v1.2 cls produces a valid PDF (no LaTeX errors).

Tests added:

• Unit tests for the pre_knit hook (warns on authormark, silent on unrelated metadata).
• A render-based test (knit-only, run_pandoc = FALSE) covering the rmarkdown→hook wiring end-to-end.
• Regression guard asserting the template no longer contains the literal \authormark string.

Fixes #603