Tri-state backend reachability (M70c).

kafkajs retries internally, so the broker runs the supervisor in observe mode: the producer's producer.disconnect/producer.connect events (the VALUES of kafkajs's producer.events.DISCONNECT / producer.events.CONNECT — the uppercase KEYS are not accepted listener names and kafkajs throws for them, X28-1) mark the fault window (consumer.crash with restart: false is terminal and surfaces as a producer.disconnect the client does not recover from). false while the window is active, true otherwise, undefined when the producer exposes no event surface (a minimal fake) — the indicator then reports reachable: 'unknown'.