The media thread will own the advisory channel (two senders: the engine + the local VAD). spawn_tap_engine now takes advisory_tx as a parameter + forwards brain speech_started/speech_stopped through it. Temporary bridge: session_map.rs's legacy poll-task call site creates a throwaway advisory channel until dev-a Task 7 (MediaThread) owns the real channel. Signed-off-by: Aaron D. Lee <himself@adlee.work>