When two runs share identical params they write to the same figs/<stem>.html,
and the most recent overwrites the earlier. Previously both got a compare
checkbox with the same data-stem, so toggling one toggled both via the JS
Set. Now we flag older duplicates server-side (first occurrence wins — Prefect
returns runs START_TIME_DESC), drop their checkbox, fade the row, and mark
'overwritten' on the outputs line.
- 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)
