Volta brackets in multiple staves

By adding the Volta_engraver to the relevant staff, volte can be put over staves other than the topmost one in a score.

\repeat and related commands should be present in all staves.

\version "2.24"

voltaMusic = \relative c'' {
  \repeat volta 2 {
    c1
    \alternative {
      \volta 1 { d1 }
      \volta 2 { e1 }
    }
  }
}

<<
  \new StaffGroup <<
    \new Staff \voltaMusic
    \new Staff \voltaMusic
  >>
  \new StaffGroup <<
    \new Staff \with { \consists "Volta_engraver" }
      \voltaMusic
    \new Staff \voltaMusic
  >>
>>