- Correct the per-generation reset comment: an in-place updater restart keeps
PTB's update_queue, so old-generation dispatches can briefly exceed
received; the check already treats that as no backlog.
- Make the once-per-stall gate explicit (cap the heartbeat count) instead of
relying on `!=`.
- Drop the dead getattr in _record_updates_received (only reachable after
_record_polling_progress dereferenced the same instance state); keep the
fallbacks in the heartbeat check and group-99 handler, which sibling
watchdogs share because object.__new__ adapter doubles exist in tests.
- Split the single invariant test so a failure names the broken guard:
stall-once, re-arm-on-progress, generation-reset; drop the unused _app mock.
dbd027287e