Function solve_varpro 

pub fn solve_varpro(
    graph: &FitGraphSpec,
    datasets: &[MeasurementSpec],
    all_params: &HashMap<String, ParameterSpec>,
    _options: &FitOptionsSpec,
) -> Result<FitResultSpec, CoreError>

Run the varpro solver on one or more datasets sharing the same separable model.

For multi-dataset fits all datasets are concatenated vertically before building the basis matrix, so nonlinear shape parameters (α) are shared across all datasets while amplitudes are solved jointly.

Errors

• CoreError::Eval if the model construction or solve fails.