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.

Document the HTTP API surface in detail, including runtime control, live config updates, split-rate behaviour, and the current hot-reload semantics for on-air parameter changes.