freshnessStream
fun <T> freshnessStream(storeData: Flow<StoreData<T>>, ttl: Duration, now: () -> Instant = { Clock.System.now() }): Flow<FreshnessBand>
Sibling stream: the per-card FreshnessBand over time, computed purely from (fetchedAt, ttl, lastError) — network state is deliberately NOT an input (a separate connectivity banner owns that). Run alongside screenStateStream to drive a freshness indicator that updates independently of the content decision.