Keep frame sequence numbers monotonic, lock the RDS carrier to the exact pilot phase used by the stereo encoder, and apply the accompanying DSP/control cleanups needed for stable live transmission behaviour.

Write PlutoSDR TX samples via per-channel buffer pointers from iio_buffer_first instead of assuming a fixed interleaved layout. Also derive the 38 kHz stereo subcarrier directly from the pilot phase to guarantee phase coherence for the FM stereo multiplex.