Function add_error_curve 

pub fn add_error_curve(
    data: &[f64],
    n: usize,
    m: usize,
    sd: f64,
    seed: Option<u64>,
) -> Vec<f64>

Add curve-level Gaussian noise to functional data.

Adds a constant noise term per curve: each observation in curve i has the same noise value.

Arguments

• data - Data matrix (n × m) in column-major format
• n - Number of samples
• m - Number of evaluation points
• sd - Standard deviation of noise
• seed - Optional random seed

Returns

Noisy data matrix (n × m) in column-major format