Another piece of valuable .metallib format documentation found only after digging for a while:

https://github.com/YuAo/MetalLibraryArchive/blob/master/README.md (Scroll down to the "Metal Library Archive Binary Layout" heading)

In addition, the Asahi Linux project has done a massive amount of work in reverse-engineering the M-series GPU instruction set. https://asahilinux.org/