Spotted a pretty mild bug related to red zones. The current code shouldn't cause any bugs. It just wastes a few bytes of stack space.
https://github.com/edubart/minicoro/blob/main/minicoro.h#L742

You subtract the 128 bytes from the size of the stack, and then use that size to find the high address of the stack. The red zone is the space with lower addresses than the current stack pointer, while your reserved space is above the start of the stack. Also, the red zone really only applies meaningfully to leaf functions so they can skip adjusting the stack pointer without getting stomped by interrupts. Any function that calls resume/yield is no longer a leaf function, so you don't have to worry about it in your implementation.