add a silent dead-link mode to the memory carrier (link stays up,
frames vanish) and a flag-gated TestDeadLinkDetectionWindow that
measures how long the datachannel client takes to notice a dead link
and reconnect. on the conservative liveness window it reconnects in
~58s; with the buggy 120s smux keepalive it fails to detect within 75s.
also drop dead restartControlKCP (both callers moved to
restartControlKCPWithHeader)
neuronori