MANUAL RUN: watch scenes (play_scene, scene_step, the queue) in a real browser

Split from #1610. Watch play_scene / scene_step, the Queue button and shift-click queue, slices like code:2-3, and the palette's Play/Queue a scene in a real browser: cursor, held steps, skip, reduced motion. Only unit tests ran.

Folded into the one manual-run list, section 9 of #609 (easiest first, with times). This issue's steps stay here; tick the item in #609.