Add a relay-server test that connects the production client-core session and the
production gateway session to the same in-process relay. The test verifies room
join catch-up, negotiated peer identities, Ethernet datagrams in both
directions, stats reporting, and room/session cleanup after graceful shutdown.
The existing relay tests covered forwarding with raw Quinn peers. This adds a
higher-level proof that the three MVP components agree on the same control and
datagram contracts before the final Windows TAP and physical LAN test.
Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay bridges_real_client_and_gateway_sessions -- --nocapture
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
Refs: PLAN.md Phase 1 relay/client/gateway proof
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