feat(gateway): open AF_PACKET sockets

The gateway now has a small Linux PacketSocket wrapper for raw Ethernet frame
I/O. It resolves the configured interface with if_nametoindex, opens an
AF_PACKET/SOCK_RAW socket for ETH_P_ALL, binds it to the interface, and exposes
thin send_frame and recv_frame helpers around the owned file descriptor.

The gateway binary opens this socket after completing the relay control
handshake. The frame bridge loop is still intentionally left for a later slice,
but the process now proves the two required resources are available: relay
admission and raw L2 access on the LAN interface.

Tests cover interface-name validation and missing-interface lookup without
requiring root or CAP_NET_RAW.

Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings

Refs: PLAN.md Linux AF_PACKET gateway socket

crates/lanparty-gateway/src/main.rs

@@ -1,4 +1,6 @@
 use clap::Parser;
+#[cfg(target_os = "linux")]
+use lanparty_gateway::PacketSocket;
 use lanparty_gateway::{GatewayArgs, connect_gateway};
 
 #[tokio::main]
@@ -19,6 +21,18 @@ async fn main() -> anyhow::Result<()> {
         gateway.welcome().effective_tap_mtu()
     );
     println!("AF_PACKET bridging is not wired yet; press Ctrl-C to stop");
+    #[cfg(target_os = "linux")]
+    let _packet_socket = {
+        let socket = PacketSocket::open(gateway.config().interface())?;
+        println!(
+            "lanparty-gateway opened AF_PACKET socket on {} (ifindex {})",
+            socket.interface(),
+            socket.interface_index()
+        );
+        socket
+    };
+    #[cfg(not(target_os = "linux"))]
+    anyhow::bail!("lanparty-gateway requires Linux AF_PACKET support");
 
     tokio::signal::ctrl_c().await?;
     gateway.shutdown("gateway shutting down").await;