Add a lock-free stdin PCM ingest path, streaming resampler, stereo-linked limiting and pre-MPX audio filtering, plus the engine/control wiring needed to drive live audio into TX mode. Also document the ingest API and include a helper batch script for piping ffmpeg audio into fmrtx.

Embed a browser-based control surface into the HTTP server and document the live-control API. The new UI exposes TX start/stop, runtime telemetry, frequency, levels, toggles, and RDS text updates against the existing live-config endpoints.

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.

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.