Type Alias OnAudioFrameCallbackType 

pub type OnAudioFrameCallbackType = extern "C" fn(RefAny, CallbackInfo, AudioFrame) -> Update;

User hook fired once per captured audio chunk - the backreference DI pattern (see architecture.md). The widget’s private writeback invokes it with each AudioFrame so application code can save it, apply effects, or send it over the network (azul-meet). Returns Update like any callback. Wired via MicrophoneWidget::with_on_frame.