Add DNS hostname support in peer addresses for UDP and TCP transports

Add resolve_socket_addr() with IP fast path and tokio::net::lookup_host()
fallback for DNS hostnames. Peer addresses can now use hostnames like
"peer1.example.com:2121" alongside IP addresses.

UDP transport adds a per-transport DNS cache (60s TTL) to avoid
per-packet resolution. TCP resolves at connect time (one-shot).

Update design docs, config examples, and changelog to reflect hostname
support in transport addressing.

packaging/common/fips.yaml

@@ -43,5 +43,5 @@ peers: []
   #   alias: "gateway"
   #   addresses:
   #     - transport: udp
-  #       addr: "217.77.8.91:2121"  # public FIPS testing node
+  #       addr: "217.77.8.91:2121"  # IP or hostname (e.g., "peer.example.com:2121")
   #   connect_policy: auto_connect

packaging/openwrt/files/etc/fips/fips.yaml

@@ -61,4 +61,4 @@ peers: []
   #   alias: "gateway"
   #   addresses:
   #     - transport: udp
-  #       addr: "1.2.3.4:2121"
+  #       addr: "1.2.3.4:2121"  # IP or hostname

packaging/systemd/README.install.md

@@ -72,7 +72,7 @@ peers:
     alias: "gateway"
     addresses:
       - transport: udp
-        addr: "217.77.8.91:2121"  # public FIPS testing node
+        addr: "217.77.8.91:2121"  # IP or hostname (e.g., "peer.example.com:2121")
     connect_policy: auto_connect
 ```