fix: settle the hero demo's stale play refusals and idle posters

Number the play attempts and let only the newest one report a refusal. The
element alone could not tell a live rejection from a leftover: a switch from
cli to tui and back leaves the cli video selected again, so a refusal from
the first attempt still passed the check and paused a pane that was playing,
leaving the next click of the control playing instead of pausing.

Hold the tui and agent posters, and the src of their fallback images, in data
attributes until the first time each pane is picked. A fresh load fetched all
three stills for one pane on screen; it now fetches the cli poster alone under
reduced motion, that poster and the cli recording otherwise, and each other
pane's still and recording when it is first selected.