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/pareto/dominance.rs

@@ -0,0 +1,147 @@
+//! Pareto dominance enum and pairwise dominance comparison.
+
+use crate::core::evaluation::Evaluation;
+use crate::core::objective::ObjectiveSpace;
+
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+/// The Pareto relationship between two evaluations.
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Dominance {
+    /// `a` dominates `b`.
+    Dominates,
+    /// `a` is dominated by `b`.
+    DominatedBy,
+    /// Neither dominates the other.
+    NonDominated,
+    /// All objective values are equal after orientation conversion.
+    Equal,
+}
+
+/// Pareto-compare two evaluations under the given objective space.
+///
+/// Behavior (spec §9.2):
+///
+/// 1. Feasible candidates dominate infeasible candidates.
+/// 2. If both are infeasible, the candidate with smaller
+///    `constraint_violation` dominates.
+/// 3. Otherwise compare objective values after converting both to
+///    minimization orientation via [`ObjectiveSpace::as_minimization`].
+pub fn pareto_compare(
+    a: &Evaluation,
+    b: &Evaluation,
+    objectives: &ObjectiveSpace,
+) -> Dominance {
+    let a_feasible = a.is_feasible();
+    let b_feasible = b.is_feasible();
+    match (a_feasible, b_feasible) {
+        (true, false) => return Dominance::Dominates,
+        (false, true) => return Dominance::DominatedBy,
+        (false, false) => {
+            return if a.constraint_violation < b.constraint_violation {
+                Dominance::Dominates
+            } else if a.constraint_violation > b.constraint_violation {
+                Dominance::DominatedBy
+            } else {
+                Dominance::Equal
+            };
+        }
+        (true, true) => {}
+    }
+
+    let am = objectives.as_minimization(&a.objectives);
+    let bm = objectives.as_minimization(&b.objectives);
+
+    let mut a_better_anywhere = false;
+    let mut b_better_anywhere = false;
+    for (av, bv) in am.iter().zip(bm.iter()) {
+        if av < bv {
+            a_better_anywhere = true;
+        } else if av > bv {
+            b_better_anywhere = true;
+        }
+    }
+
+    match (a_better_anywhere, b_better_anywhere) {
+        (true, false) => Dominance::Dominates,
+        (false, true) => Dominance::DominatedBy,
+        (false, false) => Dominance::Equal,
+        (true, true) => Dominance::NonDominated,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::core::objective::Objective;
+
+    fn space_min2() -> ObjectiveSpace {
+        ObjectiveSpace::new(vec![
+            Objective::minimize("f1"),
+            Objective::minimize("f2"),
+        ])
+    }
+
+    #[test]
+    fn a_dominates_b() {
+        let s = space_min2();
+        let a = Evaluation::new(vec![1.0, 1.0]);
+        let b = Evaluation::new(vec![2.0, 2.0]);
+        assert_eq!(pareto_compare(&a, &b, &s), Dominance::Dominates);
+    }
+
+    #[test]
+    fn a_dominated_by_b() {
+        let s = space_min2();
+        let a = Evaluation::new(vec![3.0, 4.0]);
+        let b = Evaluation::new(vec![1.0, 2.0]);
+        assert_eq!(pareto_compare(&a, &b, &s), Dominance::DominatedBy);
+    }
+
+    #[test]
+    fn non_dominated_pair() {
+        let s = space_min2();
+        let a = Evaluation::new(vec![1.0, 4.0]);
+        let b = Evaluation::new(vec![3.0, 2.0]);
+        assert_eq!(pareto_compare(&a, &b, &s), Dominance::NonDominated);
+    }
+
+    #[test]
+    fn equal_pair() {
+        let s = space_min2();
+        let a = Evaluation::new(vec![2.0, 2.0]);
+        let b = Evaluation::new(vec![2.0, 2.0]);
+        assert_eq!(pareto_compare(&a, &b, &s), Dominance::Equal);
+    }
+
+    #[test]
+    fn feasible_beats_infeasible() {
+        let s = space_min2();
+        let a = Evaluation::new(vec![10.0, 10.0]);
+        let b = Evaluation::constrained(vec![1.0, 1.0], 5.0);
+        assert_eq!(pareto_compare(&a, &b, &s), Dominance::Dominates);
+        assert_eq!(pareto_compare(&b, &a, &s), Dominance::DominatedBy);
+    }
+
+    #[test]
+    fn smaller_violation_wins_among_infeasible() {
+        let s = space_min2();
+        let a = Evaluation::constrained(vec![100.0, 100.0], 1.0);
+        let b = Evaluation::constrained(vec![0.0, 0.0], 5.0);
+        assert_eq!(pareto_compare(&a, &b, &s), Dominance::Dominates);
+    }
+
+    #[test]
+    fn maximize_axis_handled_correctly() {
+        let s = ObjectiveSpace::new(vec![
+            Objective::minimize("cost"),
+            Objective::maximize("accuracy"),
+        ]);
+        // a: cost=1 (better), accuracy=0.9 (better) → dominates b
+        let a = Evaluation::new(vec![1.0, 0.9]);
+        let b = Evaluation::new(vec![2.0, 0.8]);
+        assert_eq!(pareto_compare(&a, &b, &s), Dominance::Dominates);
+    }
+}

src/pareto/mod.rs

@@ -0,0 +1,5 @@
+//! Pareto utilities: dominance, fronts, sorting, crowding, and an archive.
+
+pub mod dominance;
+
+pub use dominance::*;