useProjection

Vue primitive that composes ChannelRegistry.acquire with Vue's reactivity system.

Contract (mirrors the React useProjection hook but adapted to Vue's setup + onScopeDispose lifecycle):

• On setup() (or whenever registry / key change) the composable acquires a ref-counted projection from the registry and subscribes to its store. Vue re-renders automatically whenever the store's snapshot changes.
• On onScopeDispose (or when registry / key change) the previous acquisition is released. If this was the last consumer of that spec, the registry closes the underlying server subscription.
• When registry is null/undefined the composable stays at initialValue. This is the happy path for root-served projections (useMessages, useToolCalls, useValues) which delegate to the always-on root store instead of acquiring a new registry projection.

registry and key accept MaybeRefOrGetter so selector composables can pass a reactive target (e.g. a computed subagent snapshot) and have the projection rebind automatically.

Returns a Readonly<ShallowRef<T>> so templates auto-unwrap and scripts can computed(() => proj.value). The underlying ref is shallowRef — consumers that mutate the snapshot get undefined behaviour; stream projections should always be treated as immutable snapshots.