Function within_threshold 

pub fn within_threshold<F>(
    p: Vector2<f32>,
    f: F,
    t_range: RangeInclusive<f32>,
    threshold: f32,
    speed_upper_bound: Option<f32>,
    options: NearestOptions,
) -> bool
where
    F: Fn(f32) -> Vector2<f32> + Copy,

Return true if the point is within threshold distance of the curve.

Uses the same conservative early-out as nearest_t_within. If the point is provably farther, returns false without running the full search. Otherwise, computes the true nearest distance and compares to threshold.