https://gitlab.com/bztsrc/posix-uefi
Its goal is twofold: provides the usual libc API for EFI applications and an SDK on POSIX systems to create such EFI applications with a build system that works with CLang and gcc as well.
Its API is nowhere near complete as GNU-EFI's or EDKII's, so it provides seamless integration with those headers to make the API expandable. It is also not a fully POSIX standard compatible libc, it's just a wrapper around the firmware provided features with some minimal additions, but provides enough for everyday use cases. As being minimal, it's extremely small (~132K of source only) and because it's dependency-free, it's easy to integrate into any project.
The repo contains lots of examples too, from a simple "Hello World" to more complex ones like displaying a PNG splash screen or loading a "kernel" in ELF format.
POSIX-UEFI is 100% AI-free, and has been tested on almost every VMs and on lots of real hardware by volunteers over the years. It is feature complete, stable and considered bug-free by now (big thanks to contributors), although some workarounds for buggy firmware might be added in the future.