Each card embeds the version-pinned Platform3 player at
/player/versions/2026-08-07.issue-2699-article-images/ and posts the
fixture file as a CONTENT_LOAD payload carrying a typed
interactiveComposition plus an inline checkpoint qtiXml.
Good cards visibly render the composition (article body or cue-labelled segment plus
a working checkpoint interaction). The broken article twin fails visibly inside the
checkpoint panel; the broken video twin is rejected at CONTENT_LOAD validation and
its posted CONTENT_ERROR is surfaced on the card.