Function volcano_plot 

pub fn volcano_plot(
    results: &DeResults,
    padj_threshold: f64,
    fc_threshold: f64,
) -> Vec<VolcanoPoint>

Convert DE results into volcano-plot points.

• padj_threshold: adjusted p-value cutoff (e.g. 0.05).
• fc_threshold: absolute log2 fold-change cutoff (e.g. 1.0).

A gene is marked significant if p_adjusted < padj_threshold and |log2_fold_change| > fc_threshold.