Covers gost#911 at the wire level: four UDP source sockets send through one
RUDP reverse tunnel bound on a relay server, and every datagram must come
back byte-identical to its own endpoint.
A frame torn by an interleaved writer shows up twice — as a corrupted
payload, and as the torn frame leaving half a header in the stream, which the
far end reads as "unexpected EOF" and answers by rebuilding the whole tunnel.
So the suite asserts both: no endpoint sees a corrupted datagram, and no
endpoint sees a receive gap long enough to be a teardown and rebind.
Verified against a binary built before the fix (4 endpoints, 35% loss, 1.4s
gaps, suite fails) and after it (0% loss, no corruption, 0.2s gaps, passes).
