Staleness degrades confidence, never fails unrelated PRs. Past the headroom window, severity thresholds shrink (a table at 600k rows three weeks ago is judged as if at the 1M tier). Past the degrade age, every row count becomes unknown → unbounded, so a stale snapshot cannot silently certify anything — but PRs with no pending migrations still pass. The age findings themselves are emitted by the snapshot_health rule, which consumes this struct.