feat(gateway): log bridged Ethernet frames

The gateway bridge now emits structured frame log lines for successful LAN to
relay and relay to LAN forwarding. Logs include the physical interface,
direction, peer id when one is known, MACs, ethertype or length field, frame
length, action, and drop reason.

This uses the shared `lanparty-obs` frame vocabulary instead of adding a second
ad hoc diagnostics model to the gateway. The log line stays local to the
gateway because the relay still owns its own room/target-specific formatting.

Test Plan:
- cargo fmt --check
- 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

@@ -124,7 +124,9 @@ control-stream hello/welcome handshake, opens an AF_PACKET socket on the LAN
 interface with promiscuous packet membership, and bridges Ethernet frames
 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.
+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.
 
 ## Windows Client