Thanks for the review!
I personally like small headers/files. I find them easier to navigate than large files. Since it seems libc++ is moving in that direction I'm happy to oblige. In this case there's another reason. Locally I've more std::format patches. Managing patch series where parts are stored in separate files makes it easier when rebasing the series. (These patches will be up for review in a later stage, for new I'm still improving them.)