The relay now sends a reliable PeerJoined control event to peers that were
already present in the room after a new peer completes the hello/welcome
handshake. Events are sent on one-frame unidirectional QUIC streams, reusing the
existing control codec without keeping the room/session lock across I/O.
Delivery is best-effort for this first lifecycle slice: a notification failure
is logged, but the newly accepted peer remains joined. PeerLeft delivery and
client-side event consumption remain separate follow-up work.
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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