Expose the transport round trip time on NetherNet channels

Channels sample the selected ICE candidate pair's currentRoundTripTime
every 3 seconds while active, through a new WebRtcSession.requestRtt
seam (default: no measurement) implemented with peer connection stats.
The value is published as NetherNetChannel#rttMillis for consumers like
Geyser's ping. It is measured by the transport's STUN checks below any
client processing, matching what RakNet's connected ping reports. The
sampler starts before channelActive fires (so a synchronous close from
a handler still cancels it) and on the already active registration
path, and is cancelled on close. Failed samples keep the last good
value; negative means no measurement yet.