Feed liveness

A stale cell has several possible causes, and each needs a different response. feedHealth() separates them, and livenessStatus(maxTickAgeMs) reduces them to one value for a circuit breaker:

Status	Meaning	Response
Unknown	The feed hasn't ticked since this implementation was deployed	Treat as no signal
Healthy	The heartbeat is alive and data is landing	Individual cells may still be stale: check those separately
GlobalStale	Ticks fire and burn gas, but nothing lands: the agents or the platform are failing	Void everything
ChainDead	The heartbeat itself is old: the schedule has stopped	Void everything, and alert an operator

Pass a small multiple of the cadence as maxTickAgeMs: 3 to 5 seconds at a one-second cadence.

The key distinction: livenessStatus reports global liveness only. A Healthy feed with one old cell means that pair is unpriced: its venues are down, it fell below the agent's minimum number of sources, or it was delisted. Void that pair; the rest of the feed is fine.

Warning

Don't use subscriptionArmed as a heartbeat. Most ways the schedule can stop are silent. The precompile unsubscribes itself when the base fee rises above the schedule's maxFeePerGas. A scheduled call can be evicted from a full Reactivity queue. An out-of-gas in the callback can leave too little gas to schedule the next tick. No event fires and no state changes, and subscriptionArmed keeps returning true for a subscription the precompile has already forgotten. lastTickMs is the field nothing else can stand in for. A consumer that trusts subscriptionArmed is worse off than one that ignores it.

feedHealth() also returns lastFailureCode, which helps diagnose a stall:

Code	Meaning
1	The request reverted on the platform
2	The request was skipped: the balance is below the deposit and subscription floor
3	A callback arrived for a request that wasn't in flight
4	The response was older than the freshness budget
5	The pair set changed while the request was in flight
6	The request finalised as something other than Success
7	The response decoded but priced no pair