I was thinking it could be simplified with two possibilities: a reference behavior that's sane and predictable; a non-portable, native, whatever behavior with w

I was thinking it could be simplified with two possibilities: a reference behavior that's sane and predictable; a non-portable, native, whatever behavior with warning that not using it means anything goes & resulting code must be expected for correctness. That can just be called Undefined. So, you get CorrectBehaviorNumberWhatever OR UndefinedIfNativeBehavior. Then, like with KCC compiler, you can have the compiler parse the program and fail by default w/ error message on any undefined behavior. These can be suppressed per module or file basis. This is what safer, system languages do. Use sane, defined stuff for almost your entire program. Hide anything hardware specific or inherently unsafe in modules labeled as such where you know it's undefined and must be thoroughly checked.