checkpoint: integrate authenticated transports

examples/client.advanced.toml

@@ -11,7 +11,7 @@ listen = "127.0.0.1:51821"
 [transport]
 mode = "auto"
 
-# Auto probing is target behavior; use only transports supported by the build.
+# Auto probing follows this order; authentication rejection stops fallback.
 [transport.auto]
 order = ["direct_udp", "quic", "http3", "tls_tcp"]
 probe_timeout_ms = 1500
@@ -26,7 +26,7 @@ port = 443
 keepalive_secs = 15
 
 [transport.http3]
-port = 443
+port = 4443
 path = "/tunnel"
 
 [transport.tls_tcp]