Function apply_angle_reference_subset 

pub fn apply_angle_reference_subset(
    angles: &mut [f64],
    network: &Network,
    bus_indices: &[usize],
    reference_bus_idx: usize,
    reference_angle0_rad: f64,
    mode: AngleReference,
)

Apply an output angle reference convention to one island/subset of buses.

The transformation is a uniform shift over bus_indices, so downstream branch flows within that connected component are unchanged.