Function createEntityBinding

Create a reactive Alpine entity snapshot for (type, id).

The returned object delegates property reads to an Alpine-reactive cell. Reading any property inside an Alpine x-effect / x-text / :bind expression causes Alpine to track the dependency. Mutations to the cell trigger Alpine's reactivity queue.

Lifecycle:

1. Register a subscriber token with the core engine (prevents GC).
2. Sync the current graph state into the cell.
3. Subscribe to future graph changes; sync on each notification.
4. Kick off an initial fetch if data is stale or absent.
5. destroy() unsubscribes from the graph and releases the engine token.

Advanced/testing use: called by the $entity magic factory.