Move the arm to the specified joint position. A partial spec is merged over the live pose;
the move is guarded against out-of-range axes (see _guarded_move_j).
Parameters:
position (Dict[int, float]) – Target joint pose. Omitted axes keep their live values.
speed_pct (float | None) – Movement speed override as a percentage (0-100). If None, uses the current
speed setting.
