Include racy reads in Rust memory model with `MaybeInvalid<T>`

black_box is a NOP opsem-wise, so it doesn't ever make sense in a story. It's either unnecessary in your story or the story doesn't quite work.

Also what's try_read? You can't just access MiniRust here. As mentioned above, the correctness argument for the "story" approach relies on the story being expressible in regular Rust code.