feat(relay): log Ethernet forwarding decisions

Phase 1 needs noisy frame diagnostics while the tunnel is being proven on real
LANs. The relay already had forwarding/drop decisions, but the runtime did not
emit the MAC-level fields needed to understand what happened to each frame.

Print one relay ingress log line for every accepted Ethernet datagram after the
room registry decides whether to forward or drop it. The line includes room,
peer id, source/destination MACs, ethertype or length, frame length, action,
drop reason, and target count using the shared diagnostics vocabulary.

This keeps logging simple stdout text for now. A later product slice can route
the same `lanparty-obs` fields through tracing or JSON logs without changing the
forwarding rules.

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

Refs: PLAN.md logging diagnostics

README.md

@@ -77,7 +77,8 @@ replies with `welcome` or `reject`, and forwards live Ethernet QUIC datagrams
 between accepted peers in the same room. It currently uses a generated
 self-signed development certificate; `--dev-cert-der-out` writes that
 certificate so the gateway and client can pin it in development. Production
-certificate handling remains future work.
+certificate handling remains future work. Ethernet forwarding decisions are
+logged with room, peer, MAC, ethertype, action, drop reason, and target count.
 
 ## Gateway