New package providing a typed Set/View/Fetcher abstraction over
atomic.Pointer[T] for periodically refreshed in-memory datasets.
Loaders take ctx and return (*T, error) so callers can cancel slow
refreshes; close-on-swap hooks let typed views release resources
deterministically. Includes Tick (with onError callback, no stderr
side-effects) and a PollFiles fetcher for local-file sources.
