Hunt the hidden boxing

Implement static long SumInterface(IEnumerable<int> values) (sums via foreach over the interface — allocates an enumerator + boxes if you use a struct enumerator through IEnumerable) and static long SumSpan(int[] values) (sums via Span<int> — zero allocations). The test proves the allocation gap: SumSpan must allocate 0 over 100 runs; SumInterface's cost is printed for comparison. Implement both, correctly, and the numbers teach the lesson.