Function solve_eht 

pub fn solve_eht(
    elements: &[u8],
    positions: &[[f64; 3]],
    k: Option<f64>,
) -> Result<EhtResult, String>

Run the full EHT calculation pipeline.

• elements: atomic numbers
• positions: Cartesian coordinates in Ångström
• k: Wolfsberg-Helmholtz constant (None = 1.75)