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. Dense vectors use the same overlap semantics as the borrowed overload.

rot over borrowed strided storage. Negative strides are supported. If the views overlap, both logical input sequences are snapshotted before writing; at a shared physical entry the final write is from y.

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.