Function brent_root_find 

pub fn brent_root_find<F>(
    f: F,
    a: f64,
    b: f64,
    x_tolerance: f64,
    max_iterations: usize,
) -> Result<AngleResult, String>
where
    F: Fn(f64) -> f64,

Brent’s method for root finding - optimized implementation.

x_tolerance is an absolute tolerance in the same units as a and b. AngleResult::final_error reports |f(x)| in the function’s output units.