Given an integer n, return the 2n rows of a centered star diamond — a Full Star Pyramid (n rows, growing) immediately followed by a Reverse Star Pyramid (n rows, shrinking). The widest row (0 spaces, 2n − 1 stars) sits in the middle, appearing twice in a row: once as the last row of the top half, once as the first row of the bottom half. Both solutions build the two halves separately and concatenate them — reusing the exact row formulas from the Full Star Pyramid and Reverse Star Pyramid problems.