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/client.yaml

@@ -0,0 +1,34 @@
+# Internal client behind NAT for gost#911: a UDP forward service that sends all
+# traffic to the port the host bound through the reverse tunnel.
+#
+# The forwarder target is gost-server:8432 — that is where the reverse tunnel is
+# actually bound (the host's rudp listener asked the relay server to bind it, so
+# the port lives on the relay server, not on the host). Datagrams go there and
+# are carried back to the internal host over the one shared tunnel stream.
+services:
+  - name: service-0
+    addr: ":8432"
+    handler:
+      type: udp
+    listener:
+      type: udp
+      metadata:
+        keepalive: true
+        ttl: 5m
+    forwarder:
+      nodes:
+        - name: tunnel
+          addr: gost-server:8432
+
+chains:
+  - name: chain-0
+    hops:
+      - nodes:
+        - addr: gost-server:8430
+          connector:
+            type: relay
+          dialer:
+            type: tcp
+
+log:
+  level: debug