M70c: delegates to the resolved broker's isHealthy?() — the only way the backplane can report the broker's reachability without importing the messaging plugin (§3.1). Assigned only when the broker provides the member: a broker that omits it is unknown, and the indicator reads absence of isHealthy (not false) as that.

M90b (X21-1): the delegation calls through the OWNER — broker.isHealthy(), not a detached const probe = broker.isHealthy reference. A broker whose isHealthy reads instance state (ServiceBusBroker's private probe cache, among others) throws a bare TypeError when invoked unbound, which a health endpoint then reports as an error rather than a fact. It retains the resolved broker's own probe cache rather than adding a duplicate backplane-level cache.