Sync-locked tracks rippled from the grabbed member's start, so grabbing a
J cut's early sound checked and shifted from before the picture's cut and
was refused. For a head trim the cut is the latest member's start. Adds the
test only the new stretch catches: a clip starting at the cut on a
sync-locked track, which the old stretch let fall out of sync.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>