checkpoint: integrate authenticated transports

examples/server.minimal.toml

@@ -1,13 +1,17 @@
 # Minimal server. Replace the backend and client secret path before use.
-# The listener address/port is an example; open only the transports you need.
+# This is a host address; the transport section below selects the port.
 
 [server]
-listen = "0.0.0.0:51830"
+listen = "0.0.0.0"
 
 [backend]
 target = "127.0.0.1:51820"
 
-# Milestone 1 direct-UDP deployment. Other transports are planned or optional.
+# Direct UDP only. Other transports can be enabled explicitly when needed.
+[transports.direct_udp]
+enabled = true
+port = 51830
+
 [transports.quic]
 enabled = false