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()