Cache altvoltage1 during Configure(), clear it in cleanup(), and use the cached channel for Tune() instead of calling findChannel on every retune. Tune() now checks the direct iio_channel_attr_write_longlong return code and reports LO write failures back to the caller with the failing frequency and libiio error code.

All major TX parameters are now hot-swappable during transmission:
- DSP params (drive, stereo, pilot, RDS levels, limiter) via
 atomic.Pointer[LiveParams], loaded once per chunk (~50ms)
- RDS text (PS, RT) via atomic.Value in encoder, applied at
 RDS group boundaries (~88ms)
- TX frequency via driver.Tune(), applied between chunks

Zero locks in DSP path. HTTP handler writes atomics, run loop
reads them. Validation happens before store.

New: SoapyDriver.Tune() for live frequency changes.
New: LiveConfigUpdate/LivePatch types for type-safe patching.
New: 4 engine tests for live DSP/freq/RDS updates + validation.

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.

Introduce CLI TX mode, hardware driver selection, IQ resampling to device rate, and platform-specific PlutoSDR/SoapySDR integrations. Update engine pacing for blocking hardware writes and refresh docs/release notes for the hardware-ready v0.6.0-pre milestone.