The R-2 mockup shows the article on a distinct light panel, but the
impl-ref table only specified the centered container, so the page
rendered flat on the canvas. Wrap the <article> in the standard card
pattern (BackButton stays outside on the canvas) and record the sheet
in both spec impl-ref tables so mockup and impl-ref agree.
Closes#797
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>