Supply bars from an existing host subscription instead of subscribing twice.
meta.provisional marks a bar whose open is only the first tick a builder
saw, see CandleUpdate.provisional. Pushed onto a bar history already
holds for that bucket, it keeps that bar's open and widens the extremes
instead of replacing them, so a repair that found the true open is not
undone by the next tick.
Optionalmeta: LiveBarMeta
Owns history, paging and live updates for one current instrument. Consumers decide how to paint snapshots; no canvas, DOM or broker execution is owned here.