writeMaintenancePass

Resumes a maintenance sequence once the user has pointed at the device's UF2 volume.

Distinct from saveDfuFile because the URI kinds differ: this takes a tree URI, which grants the sibling access needed to read INFO_UF2.TXT and vet the volume before writing. Each pass re-picks, because the device re-enumerates between passes and the previous grant no longer refers to the mounted volume.