Measure and improve the constant numbers used when building the tree

We must take three parameters into account:

1. Time to build the tree
2. Relevancy of the searches
3. Time to search in the tree

Fun fact: the lowest in the tree you are, the less impact a dummy plane has on the search cost.

https://github.com/meilisearch/arroy/blob/7fc603166af61e18e5257cf5859e6cf71ecbfeab/src/writer.rs#L248-L259

fn split_imbalance(left_indices_len: usize, right_indices_len: usize) -> f64 {
    let ls = left_indices_len as f64;
    let rs = right_indices_len as f64;
    let f = ls / (ls + rs + f64::EPSILON); // Avoid 0/0
    f.max(1.0 - f)
}

fn main() {
    dbg!(split_imbalance(29464, 18394));
    dbg!(split_imbalance(30000, 30000));
    dbg!(split_imbalance(30000, 1580));
}

Start in src/writer.rs at split_imbalance and review the constants used while building the tree. Measure the three stated concerns—tree-build time, search relevance, and search time—using the provided imbalance examples as an initial reference. The issue does not define target values, benchmarks, tests, or acceptance criteria, so the work is not ready to start without further direction.