feat(react): add stable lifecycle events (#53)

## Summary

- expose structured lifecycle events through `DomainKit.Root`
- keep the latest host callback without restarting the managed runtime
- publish events only after canonical state changes and isolate observer
failures

```tsx
<DomainKit.Root
  onEvent={(event) => analytics.track(event._tag, event)}
  transport={transport}
>
  {children}
</DomainKit.Root>
```

```text
successful mutation -> canonical state update -> best-effort host event
```

## Validation

- React package typecheck
- 54 React unit tests
- repository lint, format, and diff checks