I kind of see this as the other way around. Darwin is a superset and would itself import (and export) C. From memory this is the functionality that currently lives in Darwin.System but I’m not at my Mac right now to check.
Is it possible to define a minimal set of things that C should contain? Or is that just as difficult as defining what OS or Platform should contain? FWIW OS would be ok for me but it’s a little hand wavey - maybe it has to be though.