feat(gateway): accept iface CLI alias

PLAN.md tells LAN hosts to start the gateway with --iface, while the binary
only accepted --interface. Add --iface as a Clap alias so the documented
Phase 1 command works without changing the canonical config field name.

The README gateway example now uses the shorthand from the plan, and a focused
parse test covers the alias mapping to the same interface field.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-gateway \
  accepts_iface_alias_for_gateway_interface -- --nocapture
- cargo test -p lanparty-gateway
- cargo clippy -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/src/lib.rs

@@ -79,7 +79,7 @@ pub struct GatewayArgs {
     room: RoomCode,
 
     /// Wired LAN interface that will later be opened with AF_PACKET.
-    #[arg(long)]
+    #[arg(long, alias = "iface")]
     interface: String,
 
     /// Gateway's advertised QUIC datagram budget before relay clamping.
@@ -832,6 +832,23 @@ mod tests {
         );
     }
 
+    #[test]
+    fn accepts_iface_alias_for_gateway_interface() {
+        let args = GatewayArgs::parse_from([
+            "lanparty-gateway",
+            "--relay",
+            "127.0.0.1:443",
+            "--relay-ca-cert",
+            "relay-cert.der",
+            "--room",
+            "ROOM1",
+            "--iface",
+            "eth0",
+        ]);
+
+        assert_eq!(args.interface, "eth0");
+    }
+
     #[tokio::test]
     async fn connects_to_relay_control_stream_as_gateway() {
         let (server_config, certificate) = test_server_config();