ARCHITECTURE.md mandate ("never the shared tokio pool") finally landed.
One std::thread owns all RtcSessions exclusively; axum routes via command
channel (Register/AcceptOffer/Delete/Shutdown). The Reflex<TapAudioPipe>
wrapper is wired here on Connected via RtcSession::set_pipe. loop_driver +
rtc_session untouched (seam holds).
Signed-off-by: Aaron D. Lee <himself@adlee.work>