Function maxsim 

pub fn maxsim(
    query: &[Vec<f32>],
    doc: &[Vec<f32>],
    metric: DistanceMetric,
) -> f32

Compute the MaxSim score between a query multi-vector and a document multi-vector.

score(Q, D) = Σᵢ maxⱼ sim(qᵢ, dⱼ)

Returns 0.0 if either side is empty.