The StereoLimiter previously used instantaneous attack — gain was reduced
in the same sample that exceeded the ceiling. While this guarantees zero
overshoot, it suppresses transient peaks that the human auditory system
cannot resolve anyway, reducing perceived loudness and causing audible
gain pumping on percussive material.

Changed to a 2ms exponential attack based on psychoacoustic burst masking
research (O. Bonello, "Multiband audio processing and its influence on
the coverage area of FM stereo transmission", JAES 2007):

 - The ear-brain system needs ~50ms to resolve distortion in a signal.
 For bursts shorter than 5ms, masking thresholds increase by up to
 36 dB compared to steady-state (burst masking).

 - With 2ms attack, initial transient peaks pass through the limiter
 unattenuated and are caught by the downstream HardClip. The clip
 artifacts last <5ms (63% reduction in 2ms, 95% in 6ms), falling
 within the burst masking window.

 - The limiter no longer reacts to micro-transients that were already
 inaudible, raising average modulation level without increasing
 perceived distortion.

Signal chain interaction:

 Audio → Drive → StereoLimiter (2ms attack, 150ms release)
 → HardClip (safety net, catches the <5ms transient peaks)
 → Cleanup LPF → HardClip
 → Stereo Encode → Composite Clipper

The HardClip after the limiter remains as the compliance safety net.
Peak modulation is guaranteed by the clip, not by the limiter. The
limiter's job is average level management; the clipper handles peaks.

Release time reduced from 200ms to 150ms for slightly faster recovery
on sustained passages without audible pumping.

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.