your library looks well written and clean. thanks for sharing.
if anyone else wants to go coroutine spelunking, these were interesting to me:
https://github.com/higan-emu/libco/tree/master
https://github.com/Tencent/libco
https://github.com/hnes/libaco
https://kernel.googlesource.com/pub/scm/virt/kvm/qemu-kvm/+/...
https://tia.mat.br/posts/2012/09/29/asynchronous_i_o_in_c_wi...
https://www.cs.uml.edu/~bill/cs516/context_paper_rse-pmt.pdf