Function t_test_two_sample 

pub fn t_test_two_sample(x: &[f64], y: &[f64]) -> (f64, f64)

Two-sample independent t-test.

Returns (t_statistic, approximate_p_value). Uses Welch’s t-test (unequal variances). Returns (0.0, 1.0) when either sample has fewer than 2 elements.