Function cluster_canonicals_chars 

pub fn cluster_canonicals_chars(
    chars: &[Vec<char>],
    threshold: f64,
) -> Vec<(Vec<usize>, f64)>

Exact single-linkage clustering over pre-collected char vectors: returns each cluster (member indices, sorted) with its exact minimum pairwise ratio. O(n²) early-exit join, rayon-parallel.