I think the rules for C includes are that `"path/header.h"` looks next to the current file first, whereas `<path/header.h>` looks only in include directories.

However, given your technique of mostly hiding the new directory name from API consumers, what do you think of calling the new directory "cpython" rather than "unstable"?

The idea there would be that the "unstable ABI" eventually become known as "the CPython C API" (since it exposes a lot of CPython implementation details", while the limited API could become known as "the portable cross-implementation Python C API".

(I know, I know, you were aiming to avoid further bikeshedding on the name, but "cpython" would namespace things nicely even if a compiler does something weird with header file lookups, and helps make it clearer to CPython contributors that we still need to care about public API stability in that directory, we just don't need to care about cross-implementation portability)