Function window_topk_idxs 

pub fn window_topk_idxs(
    win: usize,
    seqlen: usize,
    start_pos: usize,
) -> (Vec<i32>, usize)

2026-09-25: Window selection ([queries][cols], cols) of slot indices, -1 for a slot holding nothing. At start_pos == 0: one causal window per query into the chunk itself. Otherwise: the whole ring, oldest first, for one query.