warmup method
Eagerly populates the cache from the backing store.
Call this once during app startup so the first read on the request hot path doesn't pay for backing-store I/O. Subsequent calls are no-ops unless the cache has been invalidated.
Concurrent calls share a single backing-store read rather than issuing multiple I/O requests.
Implementation
Future<void> warmup() async {
if (_loaded) return;
await _load();
}