Liveness, and whether the billing ticker is still turning (M5.4).

503 when the ticker has not completed a pass in 3 × TICK_INTERVAL_SEC. Three intervals, not one: it leaves room for a slow tick plus its sleep, so the alarm means "stopped" rather than "busy".

⚠️ Measured against wall time, never against ticker_heartbeat. That field is written from the platform clock, which the test suite moves by months (and which a demo panel could move, until M0). A liveness check answers "is the loop running", and that is a question about real time. ticker_heartbeat_wall exists for this and nothing else.

When no ticker runs in this process the field is unset and the check is skipped, reported as such rather than passed over in silence: a monitor aimed at a process that was never going to tick should be able to see that from the answer. Under pytest that is every process — the suite drives run_tick directly with a demo clock.

Same body in both cases, and deliberately so: a monitor that has to parse two shapes to tell healthy from stale will eventually parse the wrong one.