feat(net): accept relay hostnames

PLAN.md describes the first client flow as entering a relay domain and room
code, but the client and gateway CLIs only accepted socket-address literals.
Add a small shared RelayEndpoint parser so bare hosts default to UDP/443 while
IP literals and explicit host:port values stay supported.

The runtime configs still store resolved SocketAddr values. That keeps the
Windows route-pinning path on a concrete relay IP before TAP activation while
avoiding duplicated endpoint grammar between client and gateway. The relay
listen config reuses the same default port constant so UDP/443 has one source.

README examples now use lanparty-relay.local and document the shared endpoint
syntax.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-net
- cargo test -p lanparty-client-win \
  accepts_relay_domain_with_default_port -- --nocapture
- cargo test -p lanparty-gateway \
  accepts_iface_alias_for_gateway_interface -- --nocapture
- cargo test -p lanparty-net -p lanparty-client-win -p lanparty-gateway
- cargo clippy -p lanparty-net -p lanparty-client-win -p lanparty-gateway \
  --all-targets -- -D warnings
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md

crates/lanparty-client-win/Cargo.toml

@@ -8,6 +8,7 @@ anyhow.workspace = true
 clap.workspace = true
 lanparty-client-core = { path = "../lanparty-client-core" }
 lanparty-ctrl = { path = "../lanparty-ctrl" }
+lanparty-net = { path = "../lanparty-net" }
 lanparty-obs = { path = "../lanparty-obs" }
 lanparty-proto = { path = "../lanparty-proto" }
 tokio.workspace = true

crates/lanparty-client-win/src/main.rs

@@ -1,9 +1,4 @@
-use std::{
-    collections::BTreeMap,
-    fs,
-    net::{IpAddr, SocketAddr},
-    path::PathBuf,
-};
+use std::{collections::BTreeMap, fs, net::IpAddr, path::PathBuf};
 #[cfg(windows)]
 use std::{
     sync::{Arc, mpsc},
@@ -26,6 +21,7 @@ use lanparty_client_route::{
 #[cfg(windows)]
 use lanparty_client_tap::TapAdapter;
 use lanparty_ctrl::{ControlMessage, PeerInfo, Role, RoomCode};
+use lanparty_net::RelayEndpoint;
 use lanparty_obs::{ClientDiagnostics, RelayDiagnostics, TapDiagnostics};
 use lanparty_proto::MacAddr;
 
@@ -42,9 +38,9 @@ const CLIENT_DIAGNOSTICS_INTERVAL: Duration = Duration::from_secs(10);
     about = "Windows TAP client for the LAN party L2 tunnel"
 )]
 struct ClientArgs {
-    /// Relay UDP socket address, for example 203.0.113.10:443.
-    #[arg(long)]
-    relay: SocketAddr,
+    /// Relay DNS name or UDP socket address; bare hosts default to UDP/443.
+    #[arg(long, value_name = "HOST[:PORT]")]
+    relay: RelayEndpoint,
 
     /// TLS server name expected in the relay certificate.
     #[arg(long, default_value = "lanparty-relay.local")]
@@ -89,8 +85,13 @@ impl ClientArgs {
             None => ClientIdentityStore::new(self.identity_file)?.load_or_create()?,
         };
 
+        let relay_addr = self
+            .relay
+            .resolve()
+            .with_context(|| format!("failed to resolve relay endpoint {}", self.relay))?;
+
         ClientSessionConfig::new(
-            self.relay,
+            relay_addr,
             self.server_name,
             relay_ca_cert,
             self.room,
@@ -778,6 +779,7 @@ fn open_tap_adapter(_session: &ClientSession) {
 mod tests {
     use super::*;
     use lanparty_ctrl::{DisconnectReason, PeerInfo};
+    use lanparty_net::DEFAULT_RELAY_PORT;
     use lanparty_obs::{QuicDiagnostics, TunnelStats};
 
     #[test]
@@ -845,6 +847,22 @@ mod tests {
         assert_eq!(refreshed.ip().unwrap().to_string(), "10.73.42.51");
     }
 
+    #[test]
+    fn accepts_relay_domain_with_default_port() {
+        let args = ClientArgs::parse_from([
+            "lanparty-client-win",
+            "--relay",
+            "relay.example.test",
+            "--relay-ca-cert",
+            "relay-cert.der",
+            "--room",
+            "ROOM1",
+        ]);
+
+        assert_eq!(args.relay.host(), "relay.example.test");
+        assert_eq!(args.relay.port(), DEFAULT_RELAY_PORT);
+    }
+
     #[test]
     fn formats_relay_lifecycle_events() {
         let gateway = ControlMessage::PeerJoined(PeerInfo::new(1, Role::Gateway, None).unwrap());