tests/e2e: concurrent UDP endpoints must share a reverse tunnel intact

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).

tests/e2e/testdata/rudpbind/server.yaml

@@ -0,0 +1,16 @@
+# Public-facing relay server for gost#911: allows UDP BIND so an internal host
+# can bind a reverse UDP tunnel through it. The host asks the server to bind
+# the tunnel's UDP port, so that port lives here — on the relay server, not on
+# the host behind NAT.
+services:
+  - name: service-0
+    addr: ":8430"
+    handler:
+      type: relay
+      metadata:
+        bind: true
+    listener:
+      type: tcp
+
+log:
+  level: debug