Prove RoPE is relative, numerically

apply_rope treats each pair of dimensions as a 2-D vector and rotates it by an angle proportional to position, with a different speed per pair. The shift-invariance check is the numerical proof the exercise asks for: if the score depends only on m − n, moving both positions by the same s changes nothing. The decision that matters most for the plot is adding a second panel with q = k: a random q and k give a noisy curve where the base's effect is hard to see, while q = k shows directly how fast "the same content, further away" stops matching.