Function grouped_down_wc 

pub fn grouped_down_wc(row_offset: u32, row_end: u32) -> u32

PURE (GPU-free, unit-tested): the shrink/expand grid grid.y (m-tile count) for one chunk window [row_offset, row_end) — ceil((row_end-row_offset)/64), min 1. Matches the launcher’s wc; exposed so the chunk-boundary math is verifiable without a GPU. A full-window call (row_offset=0, row_end=te) returns exactly the pre-chunk ceil(te/64).