The reviewer flagged two gaps in the #7 coverage: no minimal
non-empty count (a 1-element [_] is the boundary the element-counter
must still get right) and no module-level array element read-back
(only .len was checked at module scope). Add local_one_len /
mod_one_len / mod_one_elem. Both still mutation-resistant: the old
collapse-to-0 reads .len as 0, not 1, so the new len rows fail it too.
42/42 dual-stage + byte-id.
Hojun-Cho
8199d6cdad