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/scripts/echo754_repro.py

@@ -0,0 +1,22 @@
+import socket, threading
+
+s = socket.socket()
+s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+s.bind(("127.0.0.1", 15738))
+s.listen(50)
+
+def loop(c):
+    try:
+        while True:
+            d = c.recv(65536)
+            if not d:
+                break
+            c.sendall(d)
+    except OSError:
+        pass
+    finally:
+        c.close()
+
+while True:
+    c, _ = s.accept()
+    threading.Thread(target=loop, args=(c,), daemon=True).start()

tests/e2e/scripts/proxy_capture.py

@@ -0,0 +1,23 @@
+# Raw-TCP backend for the PROXY-protocol e2e test (issue #677).
+# Reads the first bytes of each connection and reflects whether a HAProxy
+# PROXY protocol header was prepended by gost:
+#   - v1 ("PROXY TCP4 ...")  -> "PROXY-RECEIVED: PROXY TCP4 ..."
+#   - v2 (12-byte signature)  -> "PROXY-V2-RECEIVED"
+#   - neither                  -> "NO-PROXY"
+import socket
+
+s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+s.bind(("0.0.0.0", 5678))
+s.listen(5)
+
+while True:
+    c, _ = s.accept()
+    d = c.recv(4096)
+    if d[:12] == b"\r\n\r\n\x00\r\nQUIT\n":
+        c.sendall(b"PROXY-V2-RECEIVED\n")
+    elif d[:6] == b"PROXY ":
+        c.sendall(b"PROXY-RECEIVED: " + d.split(b"\r\n", 1)[0] + b"\n")
+    else:
+        c.sendall(b"NO-PROXY\n")
+    c.close()