4b. setjmp / longjmp

Deliberately absent, and worth stating why because it looks like an easy win.

setjmp is the single most valuable control-flow check available - it saves and restores the stack directly, which is exactly the sort of thing an emulator gets subtly wrong and nothing else in this suite exercises. It is also the one C library function whose argument is a struct of unknown size, and jmp_buf is not something to guess at (D008). Over-allocating a "surely big enough" buffer is guessing with extra steps.

What would close it: a confirmed jmp_buf size for this platform. One number.