feat(react): establish connection flow package (#29)

## Summary

- convert DomainKit into a symmetric Bun workspace with independent core
and React packages
- add the browser-safe `@domainkit/react` connection transport, provider
root, controller, semantic parts, and ready flow
- support OAuth redirect, explicit token fallback with provider-declared
parameters, existing-authorization reuse, connected, unsupported, error,
and retry states
- preserve SSR safety and host-controlled navigation without exposing
credentials or provider clients to the browser package
- prove the package through interaction tests and a clean packed Vite
and SSR consumer

## Verification

- `bun install --frozen-lockfile`
- `bun run release:check`