useProjection

React-side primitive that composes ChannelRegistry.acquire with useSyncExternalStore.

Contract:

• On mount (or when key changes) this hook acquires a ref-counted projection from the registry and subscribes to its store. React re-renders automatically whenever the store's snapshot changes.
• On unmount (or when key changes) the previous acquisition is released. If this was the last consumer of that spec, the registry closes the underlying server subscription.

The first render (before useEffect runs) returns initialValue. Subsequent renders read from the acquired store.

Framework bindings for Vue/Svelte/Angular follow the same shape — acquire on setup/mount, release on scope-dispose/unmount — but use their own reactivity primitive instead of useSyncExternalStore.