Function rotate_angles_align_then_offset 

pub fn rotate_angles_align_then_offset(
    g: &mut PhyloSplitsGraph,
    taxon_id: usize,
    target_deg: f64,
    extra_offset_deg: f64,
) -> (Option<f64>, f64)

Align the first edge incident to taxon_id to target_deg then apply a global rotation of extra_offset_deg.

Returns (align_delta, extra_offset_deg) where:

• align_delta is Some(deg) if alignment was performed, else None (e.g., if the taxon or leaf edge wasn’t found).