Frame-captured UI latency: check per take, beware later redraws

When timing UI events from screen-capture frames with a 'last change inside the target band' rule, a later full-screen redraw that happens to overlap the band gets picked as the event time. Guard against it: check that the band is blank in the frame before the event, and if a change in the band coincides with changes in rows above and below, take the first frame where the band fills instead. Report the frame spacing (e.g. ~33 ms at 30 fps) next to any gap between medians. Claim one side is faster only if it wins in every take, not just at the pooled median.