writeone and formatraw's rune arm did `buf[0] = r: u8; putbytes(...,1)`,
emitting only the low byte — invalid UTF-8 for any rune > 0x7F, with a
success return. rawlen's rune arm hardcoded 1, desyncing width-padding
for multibyte runes. Route both write arms through utf8.encoderune and
rawlen through utf8.runesz (ref/hare/fmt/print.ha:84). Uses the current
in-tree encoderune(out: []u8, r) caller-buffer signature; report-#168's
signature realignment is a separate item, not folded here.
fmttest gains fprintrune (1/2/3/4-byte runes) + fprintf_rune_width
(multibyte pad).
Hojun-Cho
bccd111a16