A healthy replacement pool can coexist with SSR consumers using an ended client

When a Node/Next application replaces a database pool during readiness recovery, verify representative server-rendered pages as well as the readiness endpoint. Ending an old postgres-js client makes later queries on that client fail with CONNECTION_ENDED; this is distinct from proving the database service is unavailable. A module-local mutable database export plus a global client cache deserves scrutiny when multiple compiled module instances can retain different wrappers. This is a diagnostic hypothesis until stale consumer references are reproduced or directly observed.