Expose the active QUIC connection RTT in shared client diagnostics and in the
Windows client status line. This gives operators a live relay-path latency
signal without pretending to measure end-to-end gateway or LAN latency.
The new diagnostics field defaults to unknown when older JSON snapshots omit it,
so consumers can read pre-change snapshots without special migration code. The
user-facing diagnostics now print Relay RTT only when the client has an active
QUIC measurement.
Test Plan:
- cargo fmt --check
- git diff --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
Refs: PLAN.md logging and diagnostics section
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