Function point_contribution_at 

pub fn point_contribution_at(
    problem: &InterferenceProblem,
    source_id: &str,
    position: Point3M,
    amplitude_at_reference: FieldAmplitude,
    phase: Radians,
    at: Point3M,
) -> Result<(f64, f64), InterferenceError>

Evaluates the outgoing point-source Green function at one point.

For distance r, this returns the Cartesian components of A * (R_ref / r) * exp(i * k_tilde * r + i * phase). A sample at or inside the problem’s singularity radius is rejected rather than clamped.