AudioGraphAgc

Loudness-driven AGC/AVC: steers BS.1770 short-term loudness toward target_lufs with slew-limited (up/down dB/s) gain inside [min, max]; adaptation freezes while momentary loudness is below gate_lufs (silence is never pumped). Also an analysis tap: emits an agc measurement (gain_db, shortterm_lufs) every adaptation tick (100 ms). All fields runtime params.