Function indexer 

pub fn indexer(
    x: &[f32],
    qr: &[f32],
    latent: Option<&[f32]>,
    seqlen: usize,
    start_pos: usize,
    offset: usize,
    ratio: usize,
    c: &IndexerCfg,
    w: &IndexerWeights<'_>,
    fc: &[(f32, f32)],
    k_cache: Option<&mut Vec<f32>>,
    shared: &mut SharedRuntime,
    is_candidate_source: bool,
    uses_candidates: bool,
) -> (Vec<i32>, usize)

2026-09-25: The indexer forward. Returns ([queries][topk], topk) compressed-position indices, offset by offset, -1 where unreachable; also publishes index keys when latent and k_cache are given (a kv source).