I think should voltas should always drop to first staff *unless* user holds control
I notice I can drop a volta to a staff other than the first staff, e.g. this second ending in the second staff:
I think that voltas by default should always be applied to the first staff (unless user explicitly holds control), because by far the most common usage would be to apply to first staff. For those rare instances where user does want to different voltas on different staves (I don't know why but conceivably someone will have a reason why), then that ability should only be accessable while holding "control", as can be done with per-staff key signatures and per-staff time signatures.
Also I am noting that generating parts from this example actually produces incorrect part for the second part, because it seems extracting parts only looks at voltas in the first staff when generating parts: disable-per-staff-voltas.mscz