Function classify_singularity 

pub fn classify_singularity(
    func: &Expr,
    var: &str,
    singularity: &Expr,
    _order: usize,
) -> SingularityType

Classifies the type of singularity at a point.

Uses the function structure to determine if the singularity is:

• Removable (limit exists)
• Pole of order n (1/(z-a)^n term)
• Essential (e.g., e^(1/z))