The zero-knob test only asserted that ack_stale still trips with the knob
at 0. Because _read_websocket_health evaluates ack age before the
event-silence dimension, the test stayed green even with the
`_event_max_silence_seconds > 0` guard deleted: it never observed the
stale stamp being ignored. Assert (True, "healthy") with knob=0, a stale
stamp and a green transport first, then make the ACK stale and keep the
ack_stale assertion. Dropping the guard now fails this test.
Also correct the on_socket_event_type comment: discord.py dispatches
socket_event_type before the op-code switch but only for a non-null `t`;
heartbeat ACK frames carry `t: null`, they do not "return before" it.
7959e3b0ff