OptionalbarsPlayback speed in bars revealed per second. Drives the play() timer
interval (1000 / barsPerSecond). Defaults to DEFAULT_BARS_PER_SECOND.
Clamped to a sane positive range on construction and in setSpeed.
OptionalonInvoked after every index change (start / step / seek / play tick / stop)
with the new virtual-end index and whether playback is currently running.
On stop() the reported index is the last replayed one and playing is
false. Hosts use this to drive a scrubber / play-pause button.
Options for ReplayController.