Function notify_observers 

pub fn notify_observers(
    observers: &mut OptObserverVec,
    variables: &SlotMap<VarKey, Box<dyn ManifoldVariable>>,
    iteration: usize,
    cost: f64,
    gradient_norm: f64,
    damping: Option<f64>,
    step_norm: f64,
    step_quality: Option<f64>,
    linear_solver: &dyn LinearSolver<SparseMode>,
)

Notify observers with current optimization state (sparse path).

This is the common observer notification pattern used by all three optimizers.