When flyingActive_, detect Space/X key transitions and emit proper flight
vertical movement opcodes so the server (and other players) see the
correct ascending/descending animation state:
- MSG_MOVE_START_ASCEND (Space pressed while flying) → sets ASCENDING flag
- MSG_MOVE_STOP_ASCEND (Space released while flying) → clears ASCENDING flag
- MSG_MOVE_START_DESCEND (X pressed while flying) → clears ASCENDING flag
- MSG_MOVE_STOP_ASCEND (X released while flying) → clears vertical state
Track wasAscending_/wasDescending_ member state to detect transitions.
Also clear lingering vertical state when leaving flight mode.