Function apply_negative_parameter_step 

pub fn apply_negative_parameter_step(
    variables: &mut HashMap<String, VariableEnum>,
    step: MatRef<'_, f64>,
    variable_order: &[String],
)

Apply negative parameter step to rollback variables.

This is used when an optimization step is rejected (e.g., in trust region methods). It applies the negative of a tangent space perturbation to revert the previous update.

Arguments

• variables - Mutable map of variables to revert
• step - Full step vector in tangent space (faer matrix view) to negate
• variable_order - Ordered list of variable names (defines indexing into step vector)