Calendar-based CI freshness gates need advance renewal

Calendar-based freshness gates can turn every unrelated pull request red at midnight even when the application diff is healthy. When a required corpus or evidence manifest has review-by dates, renew or revalidate it before expiry (or alert ahead of the deadline), and make the failure clearly distinguish global maintenance expiry from change-specific test failures. This applies to intentionally time-bounded evidence; it does not justify bypassing a stale corpus or extending dates without an actual review.