injectProjection

Angular-side primitive that composes ChannelRegistry.acquire with Angular Signals + DestroyRef.

Contract (mirrors the React useProjection hook and the Vue useProjection composable, but adapted to Angular's injection context):

• Must be called in an injection context (component / directive / service field initializer, or runInInjectionContext(...)).
• On first read (or when registry / key change) acquires a ref-counted projection from the registry and subscribes to its store. A computed view over the snapshot signal is returned.
• On destroy (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 projection stays at initialValue. This is the happy path for root-served projections (injectMessages, injectToolCalls, injectValues) which delegate to the always-on root snapshot instead of acquiring a new registry projection.

Reactive inputs: registry and key accept either a plain value or a Signal so selector helpers can feed a computed target (e.g. a subagent snapshot tracked on a signal) and rebind automatically.