useSyncExternalStore: test snapshot identity and the initial SSR value
An external store can notify its subscribers without changing the screen rendered through useSyncExternalStore. After the same snapshot object was mutated in this experiment, the screen still displayed 0 following the notification. The Hook needs a subscription notification and a snapshot that represents the change. With SSR, the initial client read must also receive the value used to generate the server HTML. This test store includes both replacement and a deliberately incorrect mutation for c…