Implement waitlist hook with signal primitives

[brkalow]

Description

This PR introduces the useWaitlist() hook, a new signal-backed hook in @clerk/react that enables developers to build custom waitlist UIs with reactive state management. It follows the pattern of useSignIn() and useSignUp(), exposing waitlist (resource), errors, and fetchStatus.

The Waitlist resource in @clerk/clerk-js has been integrated with the signal system, providing a WaitlistFuture instance for reactive updates. The State class now manages a lazily initialized, singleton Waitlist instance, ensuring its independence from the Client resource and proper signal propagation.

An integration test has been added to validate the hook's functionality, including joining the waitlist, error handling, and loading states.

To test the changes, run the new integration test:
pnpm playwright test integration/tests/custom-flows/waitlist.test.ts

Checklist

• pnpm test runs as expected.
• pnpm build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other:

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Project	Deployment	Preview	Comments	Updated (UTC)
clerk-js-sandbox	Ready	Preview	Comment	Oct 30, 2025 8:56pm

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch cursor/implement-waitlist-hook-with-signal-primitives-3ad1

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[brkalow]

The proxy layer needed a way to access a waitlist instance, and so this property was added. It would be nice if there was a way to avoid having to do this though... 🤔