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-gateway/Cargo.toml

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

crates/lanparty-gateway/src/lib.rs

@@ -27,6 +27,7 @@ use lanparty_ctrl::{
     MAX_CONTROL_MESSAGE_LEN, PeerInfo, RELAY_ALPN, Role, RoomCode, ServerWelcome,
     decode_control_frame, encode_control_message,
 };
+use lanparty_net::RelayEndpoint;
 use lanparty_obs::TunnelStats;
 #[cfg(target_os = "linux")]
 use lanparty_obs::{FrameAction, FrameDirection, FrameLog};
@@ -62,9 +63,9 @@ const MIN_ETHERNET_FRAME_WITHOUT_FCS: usize = 60;
     about = "Linux LAN gateway for the LAN party L2 tunnel"
 )]
 pub struct GatewayArgs {
-    /// 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")]
@@ -96,8 +97,13 @@ impl GatewayArgs {
             )
         })?;
 
+        let relay_addr = self
+            .relay
+            .resolve()
+            .with_context(|| format!("failed to resolve relay endpoint {}", self.relay))?;
+
         GatewayConfig::new(
-            self.relay,
+            relay_addr,
             self.server_name,
             relay_ca_cert,
             self.room,
@@ -776,6 +782,7 @@ mod tests {
     use std::time::Duration;
 
     use bytes::Bytes;
+    use lanparty_net::DEFAULT_RELAY_PORT;
     use quinn::{ServerConfig, TransportConfig, crypto::rustls::QuicServerConfig};
     use rustls::pki_types::{PrivateKeyDer, PrivatePkcs8KeyDer};
 
@@ -837,7 +844,7 @@ mod tests {
         let args = GatewayArgs::parse_from([
             "lanparty-gateway",
             "--relay",
-            "127.0.0.1:443",
+            "relay.example.test",
             "--relay-ca-cert",
             "relay-cert.der",
             "--room",
@@ -846,6 +853,8 @@ mod tests {
             "eth0",
         ]);
 
+        assert_eq!(args.relay.host(), "relay.example.test");
+        assert_eq!(args.relay.port(), DEFAULT_RELAY_PORT);
         assert_eq!(args.interface, "eth0");
     }