[miguel-heygen]

What changes

The Studio A-roll backdrop check added in #5475 now also fails if Studio stops stamping the backdrop with data-hf-autostamped. Before, it only recorded the stamp, so if Studio ever stopped stamping it, the fixture would quietly stop testing the shape it exists for (a preview-stamped element the project's own script hides).

No visible change

Test-only: one assertion in packages/studio/tests/e2e/aroll-backdrop.mjs.

Checked

• On the devbox, the check run exactly as the studio-timeline-viewport CI job runs it: exit 0 three times in a row, every reading plateStamped: "1".
• With the stamping in packages/core/src/runtime/init.ts switched off and the runtime rebuilt: exit 1 with paused-start: #plate is not stamped (data-hf-autostamped=null).
• oxfmt, oxlint and the comment citation check are clean.

[somanshreddy]

Review at c4b2e69c: no blockers, approving.

• The assertion matches what the runtime writes. init.ts stamps with setAttribute(RUNTIME_STAMPED_ATTR, "1") (lines 2065 and 2090), and RUNTIME_STAMPED_ATTR is "data-hf-autostamped". So plateStamped !== "1" checks the exact value, and an unstamped plate (null) fails with a clear message.
• It runs in CI. aroll-backdrop.mjs runs in the studio-timeline-viewport job (ci.yml, line 1035). That job ("Studio: timeline viewport gate") passed at this head. Its log shows "plateStamped": "1" on every reading and "failures": [], so the new assertion actually ran and held, not just a skip.
• The diff is tiny: one test file, +7/-1. The failure feeds the existing failures list, which exits 1. All required checks are green.

— Somu