Voltas not reachable by keyboard
Alt+Left/Right will traverse other lines/spanners, but not voltas. That's because they don't have a start element that matches the current segment. If we don't mind only hitting voltas when navigating the top staff, it should be an easy fix. Arguably they could be hit on every staff like horizontal frames are, but that's not how they look to sighted viewers, so here the easy way might be the sensible way.