Function best_split_with_scratch 

fn best_split_with_scratch(
    samples: &[(TreeSample, f64)],
    min_samples_leaf: usize,
    sse: f64,
    min_rel_var_reduction: f64,
    scratch: &mut Vec<(TreeSample, f64)>,
) -> Option<(u8, u64)>

Variant of best_split that reuses a caller-provided buffer for sorting. The buffer is cleared and filled from samples for each feature, so the per-feature allocation is avoided after the first call.