Ctrl+Shift+Left/Right move every note in the take by one grid cell, for
clips that landed off the beat.
Since the whole take moves together, notes cannot collide with each other;
the hazard is the item boundary. Notes pushed past either end stop sounding
but are not deleted, so the operation is reversible -- shifting back restores
them. The plain keys still refuse when that would happen and report the count,
because a clip that has silently lost its first bar is a bad thing to discover
later. Ctrl+Alt+Shift forces it and names the consequence.
Positions are converted through QN rather than offset in raw ticks, so shifts
stay musically correct across tempo changes.
