i64dec computed the magnitude with `n = -n` inside i64, which wraps at
i64::MIN (-MIN == MIN stays negative), so the `for (n > 0)` loop never
ran and println(i64::MIN) emitted only the bare '-'. The whole print/
println/fprint family was affected at the single boundary value (and
int MIN, since int is 8B). ref/hare/fmt/print.ha:124-129 takes the
magnitude through math::absi64 into a u64; do the same (math.absi64
exists, lib/math/math.ww:12). ww's own printf path was already correct
((-v): u64), so print and printf now agree.
fmttest gains fprinti64min + fprintintmin pinning the boundary.
Hojun-Cho
69e31355ce