feat(gateway): report tunnel stats to relay

Gateway traffic now contributes to the shared TunnelStats model. The gateway
counts Ethernet frames sent to the relay, Ethernet frames received from the
relay, incoming relay datagrams, dropped datagrams, and malformed frames.

Expose a gateway stats snapshot and send it to the relay over the same reliable
post-handshake Stats control stream used by clients. The Linux bridge loop sends
periodic stats snapshots as diagnostics; send failures are logged instead of
bringing down the bridge because stats are observational.

The gateway relay-session test now has the server decode a Stats event before
shutdown, so the stats control path is covered without a close race.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-gateway \
  connects_to_relay_control_stream_as_gateway -- --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

README.md

@@ -137,7 +137,8 @@ between the relay and wired LAN until shutdown. It tracks remote-client source
 MACs seen from relay traffic and periodically emits small CAM refresh frames so
 the physical switch keeps those MACs associated with the gateway port. Gateway
 frame logs include direction, peer id when present, MACs, ethertype/length,
-frame length, action, and drop reason.
+frame length, action, and drop reason. The gateway also tracks frame/datagram
+counters and periodically sends stats snapshots to the relay.
 
 ## Windows Client