Function mr_complexity_changed 

pub fn mr_complexity_changed(
    mr_id: &str,
    title: &str,
    old: &str,
    new: &str,
    web_url: &str,
)

Notify that an MR’s review complexity category has changed (e.g. EASY → COMPLEX).

Fires when the computed difficulty score crosses a category boundary during a refresh. old and new are human-readable labels matching the UI badges (e.g. "🟢 EASY", "🟡 MEDIUM", "🔴 COMPLEX").