feat(ff-effect): add asyncClient utility

[fdarian]

Summary

• Add asyncClient function that converts Effect-based clients to async/await clients
• Creates a ManagedRuntime from a provided layer, resolves the client effect, and wraps all methods with a recursive Proxy to return Promises
• Includes dispose() for runtime cleanup
• Exports AsyncClient type for external use

Test plan

• Single-level method wrapping
• Nested group method wrapping
• Error propagation (Effect.fail → rejected Promise)
• Dispose cleanup
• Type safety with expectTypeOf
• Build succeeds