useProjection

Svelte binding over ChannelRegistry.acquire. Mirrors the React and Vue useProjection primitives with an idiomatic Svelte shape:

• On the first $effect run the composable acquires a ref-counted projection from the registry, seeds the returned current with the current snapshot, and subscribes to the store so templates auto-update on subsequent snapshots.
• The $effect cleanup returned from the effect body runs on both scope teardown and before the next run, releasing the previous acquisition (and letting the registry close the underlying server subscription once the last consumer leaves).
• When the resolved registry is null / undefined the composable stays at initialValue. This is the happy path for selector composables that short-circuit the root namespace by reading stream.messages / stream.values directly — they don't call useProjection at all at the root. Dynamic inputs that flip from root to scoped can pass null for the root case and fall back to initialValue.

registry and key accept plain values or getters. Reading the getter inside $effect auto-tracks any $state the getter references, so the effect re-runs and re-acquires whenever the resolved target changes.

Must be called from a reactive context (a .svelte component script or inside $effect.root). Calls outside a reactive scope will not receive store updates because $effect never fires.