Column.maskIndices

The indices whose mask entry is true; a null mask entry counts as false. The walk is word-wise over the mask's packed bits: a counting pass per chunk runs as independent parallel units, an ordered serial prefix turns the per-chunk counts into disjoint output slices, and the emission pass writes each chunk's indices into its own slice - the concatenation equals the serial walk's output bit for bit, whatever the scheduling (ADR-2026-10-19-02).