e2e: add PROXY protocol test (gost#677), bump core/x deps

ProxyProtoSuite starts gost with metadata.proxyProtocol set and asserts
the HAProxy PROXY header is prepended on outbound connections (v1/v2),
backed by a raw-TCP capture script and configs. Add echo754_repro.py
from the hot-reload EADDRINUSE investigation (gost#754).

Bump github.com/go-gost/core to v0.6.1 and x to v0.16.0.

tests/e2e/testdata/proxyproto/v2.yaml

@@ -0,0 +1,18 @@
+# go-gost/gost#677 — send-proxy (PROXY protocol v2) for the rtcp forward handler.
+# Same as v1.yaml but emits the binary v2 header (metadata.proxyProtocol: 2).
+services:
+  - name: service-0
+    addr: ":8080"
+    handler:
+      type: rtcp
+      metadata:
+        proxyProtocol: 2
+    listener:
+      type: rtcp
+    forwarder:
+      nodes:
+        - name: target-0
+          addr: proxy-capture:5678
+
+log:
+  level: debug