The relay now sends a reliable PeerLeft control event to remaining room peers
after removing the departed peer from session and room state. Normal connection
closure is reported as Normal, while malformed-datagram disconnects are reported
as ProtocolError.
This completes the first relay-side lifecycle event pair. Delivery remains
best-effort and client-side event consumption is intentionally left for a
separate slice.
Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay forwards_ethernet_datagrams_between_joined_peers \
-- --nocapture
- cargo test -p lanparty-relay
- cargo clippy -p lanparty-relay --all-targets -- -D warnings
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
Refs: PLAN.md
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