StoreListener

Minimal observable store backing every framework binding.

The shape is intentionally tiny:

• subscribe(listener) → unsubscribe lines up with React's useSyncExternalStore.
• getSnapshot() returns a referentially-stable value for unchanged state so React can bail out of renders.
• Vue/Svelte/Angular bindings wrap subscribe + getSnapshot in their own reactivity primitive (shallowRef / writable / signal) in ~10 lines.

Snapshot identity matters: a listener is only useful if getSnapshot() returns a different reference when state changes. Callers MUST pass a freshly-constructed value to setState; mutating the previous snapshot in place will break React's bail-out and cause infinite re-renders.