Pilot for task #8 (runtime-N alloc API). `alloc([], n)!` yields a
slice with cap=n, len=0; explicit `buf.len = s.len;` lifts the len
before the fromutf8_unsafe reinterpret. Same shape as
ref/hare/strings/dup.ha:15 modulo ww not yet having `append`
(task #36) — open-coded byte loop in lieu of static-append.
Verified 132/132 + 995_self_rebuild byte-identity. Pattern is the
template for the next α-category sites (concat/join/lpad/rpad/...).
Hojun-Cho
4c07ef0552