read a cross-staff beam group across another voice's note without a backup in mx::impl #505

Written on behalf of the user by AI...

#492 (for #425) taught the reader that a change of <voice> text between two notes, with no <backup>, starts another voice, and taught the writer to put a voice that only plays in another voice's rests into those rests instead of after a <backup>.

One case was left as it was on main: a cross-staff beam group in a voice whose rests another voice fills.

MeasureReader::scanForCrossStaffBeamGroups (added for #289) runs before the notes are read and tracks open level-1 beam groups by the count of <backup>s so far, not by voice. It does not look at <voice> text. So when a note of voice 5 sits between two beamed notes of voice 4 with no <backup>, the scan sees voice 4's group broken, the same way a <forward> breaks it. The group is not recognized as cross-staff, and its notes are filed under the staff each one names instead of the voice's home staff.

What needs to be done:

• Track open beam groups per staff and per <voice> text within a <backup> stretch, matching how findVoiceBucket now splits voices, so a note of another voice does not break the group.
• Add a test: a two-staff measure where voice 1 has a beam group crossing to staff 2, and voice 2 plays in a rest inside that group with no <backup>. Read it, check the group's notes share one home staff, write it and read it back.

Related: #492, #425, #289, #422