Function compute_effective_mass 

pub fn compute_effective_mass(
    inv_mass_a: f64,
    inv_mass_b: f64,
    inv_inertia_a: [f64; 3],
    inv_inertia_b: [f64; 3],
    j: [f64; 12],
) -> f64

Compute the effective mass (inverse denominator) for a constraint.

inv_mass_a, inv_mass_b — inverse masses. inv_inertia_a, inv_inertia_b — diagonal inverse inertia tensors (3 values each). j — Jacobian row (12 values from compute_jacobian).

Returns the effective mass = 1 / (J M⁻¹ Jᵀ).