write_str loop should terminate

i've got a sneaking suspicion that in the serial context the ANSI control characters are taking up invisible space, which definitely makes #493 look way goofier than it should: <img width="1047" height="185" alt="image" src="https://github.com/user-attachments/assets/4367103c-9be1-4360-84ff-9d0c5658f2f3" /> but at least with this the above doesn't send the kernel off into a spin? anyway this seems to do the needful for #550.