The arc-border running indicator animated background-position (repaint
every frame: ~3,600 main-thread style recalcs/min per arc, ~4,100ms/min
of renderer task time measured over 60s of true idle) and progress-slide
animated left (forced layout every frame). Both now travel via transform
on the compositor: same visuals, 3,617 -> 79 style recalcs/min and
4,108 -> 153 ms/min task time in the same harness (-96%).
443d4387b5