Function fit_with_scratch 

pub fn fit_with_scratch(
    samples: &[TreeSample],
    max_depth: usize,
    min_samples_leaf: usize,
    max_nodes: usize,
    min_rel_var_reduction: f64,
    scratch: &mut FitScratch,
) -> SerializedTree

Fit a tree reusing the caller-provided scratch arena. After the first call the arena retains its capacity, so subsequent fits do not allocate. The logic is identical to fit(), only the temporary buffers are reused.