TESTING.md asks for a lifecycle sanity check where the Windows client stays
connected while the LAN gateway restarts. The relay integration tests covered a
single gateway/client lifetime, but they did not prove that a replacement
gateway receives the existing client catch-up state and can bridge frames after
the first gateway leaves.
Add a real-session relay test that connects a gateway and client, shuts down the
first gateway, verifies the client receives the gateway leave event, connects a
second gateway, verifies both sides receive the expected lifecycle state, and
then sends Ethernet frames in both directions through the replacement gateway.
Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay reconnects_gateway_while_client_stays_joined \
-- --nocapture
- cargo test -p lanparty-relay
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
Refs: MVP lifecycle sanity check
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