Explore union types in C# 15

C# 15 introduces the `union` keyword in .NET 11 Preview 2, allowing developers to declare a closed set of case types with compiler-enforced exhaustive pattern matching. Unlike object, marker interfaces, or abstract base classes, union types don't require related types and prevent external extension. The compiler generates a struct with implicit conversions from each case type and a `Value` property. Switch expressions over union types are exhaustive without needing a discard arm, and adding a new case type triggers compiler warnings on incomplete switches. Custom union types are also supported via a `[Union]` attribute for existing libraries, including a non-boxing access pattern for performance-sensitive scenarios. Related proposals for closed hierarchies and closed enums round out a comprehensive exhaustiveness story for C#.