Guard Clauses that Accumulate

Implement Validate with guard clauses. Unlike fail-fast validation, this one **accumulates every violation in a fixed order**: 1. order itself null → "order" 2. Customer null/empty → "customer" 3. no items → "items" 4. each negative-qty item, in order → "qty:" + index (0-based) Valid input → an empty list. Guard clauses keep each rule on one line; the rules still *all* run (except when a earlier rule makes a later one meaningless — null order has no items to inspect). ``csharp public static class Solution { public static System.Collections.Generic.List<string> Validate(Solution.Order? order); } ``