Neither Twilio nor Telnyx documents WS keepalive — keepalive is entirely
our job (spec §3.1 invariant 3), and the caller→engine direction can be
the only traffic for hours, letting any 60s proxy idle timer kill a live
call. Engine-originated Ping frames as a third select! branch in the
existing pump loop; interval via the config.rs fail-fast parser pattern
(0 rejected — no 'off' spelling by design).
Router signature grows the interval; both call sites updated. WebRTC
signaling is plain HTTP today, so this is the only long-lived WS.
Signed-off-by: Aaron D. Lee <himself@adlee.work>
Aaron D. Lee
ac3670b962