Debug: The Exception That Vanished

SaveAllAsync fires SaveAsync for each item but never awaits the tasks — one throws and the failure disappears (the harness detects faults left unobserved). Fix: fan out AND observe every completion, rethrowing the first failure after all settle. ``csharp public sealed class SaveTrap { public SaveTrap(params string[] failingKeys); // provided public Task SaveAsync(string key); // provided: throws for failingKeys } static Task SaveAllAsync(SaveTrap trap, IEnumerable<string> keys); ``