[Feature]: Support C# 15 closed classes (AreClosed / BeClosed) #519

Description

C# 15 (.NET 11) adds closed classes. These are class hierarchies whose subclasses must all be declared in the same assembly, so a switch over them can be exhaustive:

public closed class Shape;
public sealed class Circle : Shape { public double Radius { get; init; } }
public sealed class Square : Shape { public double Side { get; init; } }

ArchUnitNET already loads these correctly. Shape shows up as an abstract Class, and both inheritance and the type checks inside switch expressions are found. However, rules can't tell a closed class apart from an ordinary abstract class.

The compiler marks the class with [System.Runtime.CompilerServices.IsClosedTypeAttribute].

Proposal, following AreSealed() / AreRecord():

• an IsClosed property on Class
• the predicates AreClosed() / AreNotClosed() and the conditions BeClosed() / NotBeClosed() on classes

I'd be happy to implement this and open a PR if the proposal sounds good to you.

Use Case(s)

// Event hierarchies must stay exhaustively matchable
Classes().That().AreAbstract().And().ResideInNamespace("MyApp.Domain.Events")
    .Should().BeClosed();

// Subclasses of closed classes are sealed
// (AreAssignableTo also matches the closed class itself, hence AreNotClosed)
Classes().That().AreAssignableTo(Classes().That().AreClosed())
    .And().AreNotClosed().Should().BeSealed();