Playback Clock

Where a playing timeline is now, as a tiny external store outside React: playbackPositionAt for the maths, createPlaybackClock advancing on requestAnimationFrame only while subscribed and the document is visible (and stopping at the end of a timeline that does not loop), usePlaybackClock to keep one clock per timeline, and usePlaybackPosition for the one leaf that draws a playhead. Code that writes to a non-React view subscribes directly, so playback never re-renders a face.