Add experimental single-sideband and vestigial-sideband stereo encoding
modes alongside the standard DSB-SC stereo subcarrier path.

- add encoder mode selection (DSB, SSB, VSB)
- add Hilbert-transform based quadrature path for SSB generation
- add vestigial split with low-band DSB and high-band SSB handling
- wire the mode through config, control, engine, generator, and fmrtx
- add test coverage for the SSB path

The default mode remains the standard DSB stereo encoder.

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.