Conformance and new members

This is with reference to `conformance.md`.

_Originally posted by @KalleOlaviNiemitalo in https://github.com/dotnet/csharpstandard/issues/1486#issuecomment-3587449267_

```csharp
using static A;

namespace B {
using static System.Int32;
class C {
static int Main() => MaxValue;
}
}

class A {
public const int MaxValue = 42;
}
```

> Now if the implementation extends System.Int32 with a `public const int MaxValue` member, then Main() will return that rather than A.MaxValue, thus changing the behaviour. So the rule seems to pretty much disallow providing additional static members. Or is this kind of program not strictly conforming?