Function big_o_normal_form 

pub fn big_o_normal_form(expr: &Expr) -> Result<Expr, AsymptoticAnalysisError>

Compute the Big-O normal form of an expression.

This is a two-phase pipeline:

1. canonical_form() — exact symbolic simplification
2. Asymptotic projection — drop dominated terms and constant factors

Returns an expression representing the asymptotic growth class.