Function layer 

pub fn layer<S>(gate: AuthGate, router: Router<S>) -> Router<S>
where
    S: Clone + Send + Sync + 'static,

Layer the auth gate onto an axum Router, independent of the router’s own state type. Callsites in each router builder use this to share one implementation. The gate is wrapped in Arc once so every route on the router observes the same snapshot, even after post-startup env-var changes.