Function expand_selection 

pub fn expand_selection(
    selected: &[i32],
    pools: &Pools,
    valid_candidates: &[u8],
    valid_keys: &[u8],
    q_positions: &[usize],
    q_mask: &[u8],
    dims: DsaDims,
    seq: usize,
    select_k: usize,
) -> Vec<i32>

2026-09-25: Expand selected pools into raw token indices, append the visible tail, and pad with INVALID. Every row is written for all out_width entries: the output starts filled with INVALID, and a padded query row stays that way.