Function build_af_filter 

fn build_af_filter(settings: &AudioSettings) -> String

Build the full MPV af (audio filter) value from the audio settings.

Combines the equalizer peaking bands and the loudness-normalization filter into a single lavfi graph, because MPV exposes one af property. The normalizer runs after the EQ so it levels the post-EQ signal. Returns an empty string when neither feature contributes a filter, which clears af.

TRACES: UR-027, UR-033 | IR-020, DR-036