After a peer is registered, later accepted-handshake failures clean up room and
session state. If the welcome was already sent, the joining peer could otherwise
keep a live QUIC connection briefly while the relay has removed it from room
membership.
Close the connection after accepted-handshake cleanup so the peer learns the
handshake failed instead of continuing with stale local state.
Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
- git diff --cached --check
Refs: MVP lifecycle ordering
ddidderr
37293b9999