feat(pareto): add Dominance enum and pareto_compare

Implements spec §9.1–9.2:

- `Dominance` enum (Dominates / DominatedBy / NonDominated / Equal).
- `pareto_compare(a, b, objectives)` with the four-step rule: feasible
  beats infeasible; among infeasibles, lower violation wins; among
  feasibles, compare in minimization orientation via
  ObjectiveSpace::as_minimization.

Unit tests cover dominance, dominated-by, non-dominated, equal, the
feasibility tiebreak, and that Maximize objectives are handled correctly
(spec §18.2).

src/prelude.rs

@@ -10,3 +10,5 @@ pub use crate::core::{
 };
 
 pub use crate::traits::{Initializer, Optimizer, Variation};
+
+pub use crate::pareto::{Dominance, pareto_compare};