Virtual-scroll synthetic

Packs N tiles, then scripts a constant-velocity scroll from top to bottom of the content while sampling every requestAnimationFrame. Reports frame-time distribution (dropped frames = frames > 20ms at 60Hz or > 10ms at 120Hz), active tile count at each sample, mount/unmount rate, and total wall time for the scroll pass. Detects regressions in the virtual pool: rising frame times or active-count drift both show up here.