Rationale
A task whose last checkpoint is older than its profile's interval is reported, not stopped.
Required behaviour
A task whose newest checkpoint is older than its profile's interval is reported as stale, and the report does not stop the work.
Failure behaviour
A violation is a WARN finding under the category checkpoint; the command continues and exits as it otherwise would. Under watch it is reported as drift and the command exits 11.
Verification
mj_validate_checkpoint decides it, dispatched from check, watch. The behavioural case test/cases/04_start_check.sh proves it, and CI runs that case.