Remove the cover art disclaimer

[tgilbert14]

Summary

• remove the visible editorial-illustration label from the Pages and in-app covers
• retain descriptive alt text, artwork provenance, and the scientific limits
• remove caption-only CSS
• add regression checks so the ornamental disclaimer does not return

Verification

• cover, handler, R parse, JavaScript, shell, responsive, accessibility, and diff gates pass
• the exact pinned manifest will be promoted from the validated PR artifact before merge