Create an animation of a shadow moving diagonally down the stairs on a transparent background. The shadow should start at the top right and gradually move down and to the left, descending the stairs smoothly. The movement should look natural and fluid, as if cast by a person walking down the steps. The shadow should subtly change in shape, maintaining a realistic perspective as it descends. Only the shadow should be visible, with no other objects or background elements, and the background must be transparent. The lighting should be soft, and the shadow should be clear and defined, showing the gradual change in direction with smooth motion