Deferred, Not Cached

Double is an iterator over a counting source. Implement Double(IEnumerable<int>) (each value ×2) and RunOnce(IEnumerable<int>) that materializes the doubled values with ONE pass over the source. The tests prove the source is pulled only when enumerated — and only once. ``csharp public sealed class CountingSource : IEnumerable<int> // provided by tests { public int ReadCount { get; } // items actually pulled } static IEnumerable<int> Double(IEnumerable<int> source); static List<int> RunOnce(IEnumerable<int> source); // exactly one pass ``