Function run_fixpoint 

pub fn run_fixpoint<F>(original: &str, enabled: bool, step: F) -> Result<String>
where
    F: FnMut(&str) -> Result<String>,

Run step until the output is a byte fixpoint or the cap is hit.

A cycle (including A/B) or a cap miss returns original. enabled false runs step once. Public so tests can inject a cycling step.