untilWhen<U> method
Waits until the reactive value equals a specific value.
Parameters:
predicate: The value to wait for
Returns: A Future that completes when the value equals predicate
Example:
final status = Signal('loading');
await status.untilWhen('ready'); // Waits until status is 'ready'
Implementation
Future<T> untilWhen<U>(U predicate) => until((value) => value == predicate);