The message listener was attached only when the mux consumer called onMessage — which
happens inside attach(), AFTER the async mTLS-registry lookup. On loopback the agent's FIRST
heartbeat ping arrived in that gap and ws dropped it (no listener), so the agent's heartbeat
died on the first miss at 15s and the tunnel flapped forever. Attach the listener at
construction and buffer early frames (and an early close) until the consumer wires up.
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