Function return_mapping_batch 

pub fn return_mapping_batch(
    pts: &mut SoaMaterialPoints,
    e: f64,
    nu: f64,
    h: f64,
)

Radial return mapping for isotropic J2 plasticity (batch, linear isotropic hardening).

Updates both the stress and accumulated plastic strain in place.

Algorithm (per point):

1. Elastic trial stress (already in pts.stress_*)
2. Compute trial Von Mises: σ_vm_trial
3. If trial < yield: elastic step, do nothing
4. Else: plastic correction — scale deviatoric back to yield surface

• e, nu — elastic constants
• h — isotropic hardening modulus (H = dσ_y / dp̄)