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Client counters were only local diagnostics even though the control protocol and relay now understand Stats messages. Add a client-core sender that opens a peer-to-relay unidirectional stream with the current TunnelStats snapshot. The Windows client reports stats whenever it prints its diagnostics snapshot. Failures are logged instead of stopping the frame pump because stats reporting is diagnostic and should not be the reason a live tunnel goes down. The client-core integration test now has the server decode the stats stream before shutdown so the send path is covered without a timing race. Test Plan: - cargo fmt --check - cargo test -p lanparty-client-core \ connects_to_relay_control_stream_as_client -- --nocapture - cargo test -p lanparty-client-core - cargo test -p lanparty-client-win - cargo clippy -p lanparty-client-core --all-targets -- -D warnings - cargo clippy -p lanparty-client-win --all-targets -- -D warnings - cargo test --workspace - cargo clippy --workspace --all-targets -- -D warnings - git diff --check Refs: PLAN.md