Function grouped_down_wc 

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

2026-09-25: grid.y of both grouped-fold kernels for the window [row_offset, row_end): ceil((row_end - row_offset) / MLG_M_TILE), at least 1. It is the same expression as the launcher’s wc, kept separate so tests can check the window math without a GPU.