docs: useSyncExternalStore pattern undocumented for consumers

[panz3r]

Context

Discovered during development of the showcase example (PR #252).

Problem

EnhancedAuthClient exposes subscribe and getSnapshot methods (compatible with React's useSyncExternalStore), but no existing example or README mentions this pattern. Without it, components only re-render via useAsyncCallback loading-state changes, which is fragile for multi-component layouts — auth state changes may not propagate correctly.

Impact

Consumers are unaware of the recommended pattern for subscribing to auth state in multi-component layouts, which can lead to subtle UI bugs where components don't re-render on auth state changes.

Proposed Action

• Add a usage example of useSyncExternalStore with EnhancedAuthClient to the core library README.md.
• Consider adding a dedicated section in the docs covering reactive auth state consumption patterns.