Libraries shouldn't decide where errors go. Tick now passes Load
errors to onError (nil to ignore); callers pick log/count/page.
check-ip supplies its own stderr writer.
Distilled from the previous net/dataset experiment and the inline
closure version in check-ip. Keeps what actually earned its keep:
- Group ties one Fetcher to N views; a single Load drives all swaps,
so shared sources (one git pull, one zip download) don't get
re-fetched per view.
- View[T].Value() is a lock-free atomic read; the atomic.Pointer is
hidden so consumers never see in-flight reloads.
- Tick runs Load on a ticker with stderr error logging.
Dropped from the v1 design: MultiSyncer (callers fan-out inline when
needed), Close (unused outside geoip), Name (callers wrap the logger),
standalone Dataset type (Group with one view covers it), Sync vs Init
asymmetry (Load handles first-call vs update internally).
check-ip rewires to use it — file/git/http modes all build a Group
with two views, uniform shape.
