Add `is_set_volume_action` to PendingQueueAction (set for CtrlSrvrSetVolume) and
complete it on the SrvrCtrlVolumeChanged echo, so rapid volume changes can
supersede the in-flight one instead of colliding on the single pending slot (the
"pending queue action already active" spam). The frontend adapter clears the
superseded pending before sending the next SetVolume (lands with the qbz-slint
controller changes).
cc
vicrodh