Apply a plane rotation (BLAS drot). Each pair (x_i, y_i) becomes (c*x_i + s*y_i, c*y_i - s*x_i), so both x and y are overwritten in place.

Generate the plane rotation that zeroes b against a (BLAS drotg), rescaling so squares that would overflow or vanish still rotate correctly. Netlib sign convention; the all-zero pair gives the identity.