The Cmd+K split ran the plain per-block split for every selected clip,
so an originally linked audio/video pair came out with its front halves
linked but the two rear halves unlinked — dragging one rear clip left
its mate behind and Link/Relink could not repair it.
Split the whole target set as ONE BlockSplitPreservingLinksCommand (the
same command the multicam path uses): any pair of originally linked
blocks split at the same time gets its new halves linked too. Adds a
gpui test asserting front and rear halves of a dropped A/V clip stay
linked after split_at_playhead.
Mike-Solar
a56f6ddf87