Gather Spans Like writev

POSIX-flavored design. Given struct span { const char *base; size_t len; };, implement size_t spans_total(const struct span *s, size_t n) (sum of lens; NULL s with n>0 returns (size_t)-1) and void spans_gather(const struct span *s, size_t n, char *out) (concatenate the spans into out in order; assume out is large enough — but skip NULL bases).