Come up with a model w.r.t derived interfaces

This issue arose from PR #[680](https://github.com/dotnet/csharpstandard/pull/680): “Define term "derived interface"”

On the TG2 call of 2026-05-06, we closed that PR, as what we think we need instead is an issue for someone to think about the model. This should be pre-C#-9, but doesn't need to block C# 8. That work should consider `static abstract` and `static virtual`, even though those won't come up until V11. See [MS proposal](https://github.com/dotnet/csharplang/blob/main/proposals/csharp-11.0/static-abstracts-in-interfaces.md) and Draft PR #1609.