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

README.md

@@ -81,5 +81,5 @@ cargo run -p lanparty-gateway -- \
 ```
 
 The gateway currently connects to the relay as `role = gateway`, completes the
-control-stream hello/welcome handshake, and then waits for shutdown. AF_PACKET
-capture and injection are not wired yet.
+control-stream hello/welcome handshake, opens an AF_PACKET socket on the LAN
+interface, and then waits for shutdown. The frame bridge loop is not wired yet.