feat(client): report relay RTT diagnostics

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

README.md

@@ -59,6 +59,7 @@ Platform-neutral remote client relay session:
 - client hello with room, virtual MAC, and datagram budget
 - welcome/reject handling with assigned peer id and effective TAP MTU
 - QUIC DATAGRAM support and negotiated datagram budget diagnostics
+- relay RTT diagnostics from the active QUIC connection
 - reliable relay control-event reads for peer lifecycle messages
 - Ethernet frame send/receive helpers over QUIC DATAGRAM with budget checks and
   local drop outcomes for malformed or oversized sends
@@ -222,15 +223,16 @@ if the driver-reported MAC does not match the tunnel identity, because an
 already-initialized Windows TAP adapter may need to be disabled/enabled or
 reinstalled before it reloads the configured `NetworkAddress`.
 It prints and reports client diagnostics snapshots with relay reachability,
-LAN-gateway presence, route-pinning, QUIC datagram budget, TAP status/IP,
-broadcast frame flow, frame/datagram counters, and drops. The periodic
-diagnostics refresh the TAP unicast IP so DHCP results that arrive after
-bridging starts become visible in later status lines. Each snapshot also emits
-short user-facing lines such as relay/gateway connection status, relay-route
-and TAP readiness warnings, DHCP address presence, and broadcast-flow
-confirmation when those signals are observed. Malformed TAP frames, jumbo
-frames, and TAP frames whose encoded datagrams exceed the negotiated QUIC budget
-are counted and dropped before relay send without stopping the bridge.
+LAN-gateway presence, route-pinning, QUIC datagram budget, relay RTT, TAP
+status/IP, broadcast frame flow, frame/datagram counters, and drops. The
+periodic diagnostics refresh the TAP unicast IP so DHCP results that arrive
+after bridging starts become visible in later status lines. Each snapshot also
+emits short user-facing lines such as relay/gateway connection status,
+relay-route and TAP readiness warnings, DHCP address presence, relay RTT, and
+broadcast-flow confirmation when those signals are observed. Malformed TAP
+frames, jumbo frames, and TAP frames whose encoded datagrams exceed the
+negotiated QUIC budget are counted and dropped before relay send without
+stopping the bridge.
 Relay lifecycle events are logged as they arrive, including gateway joins and
 peer leaves. The client remembers peer identities from join and catch-up events
 so later leave logs can identify a disconnected LAN gateway or client MAC when