Tri-state backend reachability (M70c).

nats reconnects itself, so the broker runs the supervisor in observe mode: Disconnect/Reconnect events mark the fault window, and the probe is isClosed() === false and rtt() resolving. true when both hold, false when the window is active or the connection reports closed, undefined when the injected client exposes neither member (a minimal fake) — the indicator then reports reachable: 'unknown'.