- per-run checkbox when embedding HTML exists; cap at 2 selected
- sticky 'compare selected' button opens /compare?a=&b= in new tab
- selection state persists across the 3s htmx poll via a Set keyed by stem
- /compare stub validates stems, renders scaffolding (three.js UI next)
Client-side: disable the form submit button until the picker writes a
dataset_id. Server-side: reject posts where dataset_id is present but
empty, instead of silently defaulting to make_s_curve.
