Function merge_block_candidates 

pub fn merge_block_candidates(
    cand_val: &[f32],
    cand_idx: &[u32],
    k: usize,
    masked: Option<&dyn Fn(u32) -> bool>,
) -> Vec<TopKItem>

Merge per-block GPU candidates (num_blocks × k pairs) into the global top-K. Blocks cover disjoint index ranges, so candidate ids are unique. Drops −∞ entries and any id in masked (the governance/sieve veto — “a masked token never returned”).