Sequential vs Concurrent: Prove It

Two ways to await three simulated fetches. SumSequential awaits one at a time; SumConcurrent starts all three, then awaits together. The simulator logs an event per transition — prove the difference by EVENT ORDER, not the clock: ``csharp public sealed class EventIo { public EventIo(); // provided public List<string> Log { get; } // "start:N" / "done:N" entries public Task<int> FetchAsync(int step); // provided: logs start, yields, logs done } static Task<int> SumSequential(EventIo io); // sum of steps+1 static Task<int> SumConcurrent(EventIo io); // same total, different order ``