Implement optimisticUpdate(store, mutate, fetch)

Implement optimisticUpdate(store, mutate, fetch)

The pattern behind every "the UI updates instantly, before the server confirms" interaction. This is the data-layer mechanics only, no React involved. Write optimisticUpdate(store, mutate, fetch) where store is a plain mutable object:

1. Apply mutate(store) immediately, synchronously, before awaiting anything.
2. Call await fetch() to persist the change.
3. If fetch() resolves, keep the optimistic state as-is.
4. If fetch() rejects, roll store back to exactly what it was before step 1, then re-throw the error.

const store = { count: 0 };
await optimisticUpdate(store, (s) => { s.count += 1; }, () => api.increment());
// store.count is 1 immediately; rolled back to 0 if api.increment() rejects