feat(gateway): bridge relay and LAN frames

The gateway now runs the actual frame bridge after relay admission. It registers
the AF_PACKET socket with Tokio using AsyncFd, reads valid LAN Ethernet frames
and forwards them as relay datagrams, and writes valid relay Ethernet datagrams
back to the LAN socket.

The packet socket is opened nonblocking so the bridge can shut down cleanly on
Ctrl-C without leaving a blocking recv thread behind. Existing send_ethernet and
recv_ethernet helpers now share the same validation and encoding helpers used by
the bridge.

This still needs a privileged LAN-host smoke test with a real wired interface,
but the compile-time and loopback coverage now include the gateway relay side of
the bridge and the non-root-safe packet-socket validation.

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

Refs: PLAN.md gateway AF_PACKET to relay bridge loop

README.md

@@ -80,7 +80,7 @@ cargo run -p lanparty-gateway -- \
   --interface eth0
 ```
 
-The gateway currently connects to the relay as `role = gateway`, completes the
+The gateway connects to the relay as `role = gateway`, completes the
 control-stream hello/welcome handshake, opens an AF_PACKET socket on the LAN
-interface, and has relay Ethernet datagram send/receive helpers. The frame
-bridge loop is not wired yet.
+interface, and bridges Ethernet frames between the relay and wired LAN until
+shutdown.