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/lib.rs

@@ -4,6 +4,9 @@
 //! will add the AF_PACKET capture/injection loop that feeds Ethernet frames into
 //! this established QUIC session.
 
+#[cfg(target_os = "linux")]
+mod packet;
+
 use std::{
     fs,
     net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr},
@@ -20,6 +23,9 @@ use lanparty_ctrl::{
 use quinn::{ClientConfig, Endpoint, crypto::rustls::QuicClientConfig};
 use rustls::pki_types::CertificateDer;
 
+#[cfg(target_os = "linux")]
+pub use packet::PacketSocket;
+
 const MAX_CONTROL_FRAME_LEN: usize = CONTROL_LENGTH_PREFIX_LEN + MAX_CONTROL_MESSAGE_LEN;
 
 #[derive(Debug, Parser)]