Function watch_encode_calls 

pub fn watch_encode_calls(beat: impl Fn() + 'static) -> EncodeCallWatch

Calls beat on this thread before every encoder and muxer call a sink makes, until the guard drops.

Some calls wait forever (h264_vulkan waits on fences with UINT64_MAX; write(2) to a stuck hard-mounted share retries forever) and cannot be interrupted, so the driving thread reports where it is and its owner decides when a call is stuck. The audio encoder’s thread is covered by the call that waits on it.