test(pareto,metrics,selection): pin shared-utility comparisons and arithmetic

Phase 1, tier 3 of the mutation-testing campaign — the shared Pareto /
metric / selection utilities used by every multi-objective algorithm.
A scoped cargo-mutants run found 75 survivors across these files; the
tests below target them.

- metrics/hypervolume.rs: dominates() boundary cases, non_dominated_
  projection retained-set pins, hso_recursive 1-D/2-D base cases,
  hypervolume_nd_from_evaluations empty/non-dominating skips.
- selection/tournament.rs: challenger_wins across the full feasibility
  cross-product + equal-objective tie; better_by_objective and
  better_by_feasibility branch pins; stochastic_ranking_select pf=0
  feasibility ordering and count-wraps-modulo-population.
- pareto/crowding.rs: exact interior crowding distance on symmetric
  and asymmetric fronts (pins the (next-prev)/span arithmetic).
- pareto/sort.rs: three-non-dominated-then-one-dominated and a strict
  3-chain producing three singleton fronts.
- pareto/dominance.rs: trade-off → NonDominated, better-on-one-equal-
  on-other → Dominates, identical → Equal.
- pareto/archive.rs: truncate boundary, trade-off kept alongside,
  equal candidate rejected, smaller-violation infeasible eviction.
- pareto/front.rs: best_candidate keeps the first of tied minima.
- metrics/spacing.rs: exact spacing for a varying-NN-distance front.

src/core/problem.rs's lone survivor (decision_schema default body
'replace with vec![]') is an equivalent mutant — Vec::new() and vec![]
are identical — and is left in the residue.

src/pareto/archive.rs

@@ -265,4 +265,62 @@ mod tests {
         a.extend(vec![cand(1, vec![1.0, 4.0]), cand(2, vec![3.0, 2.0])]);
         assert_eq!(a.members().len(), 2);
     }
+
+    /// `truncate` keeps the archive untouched when it is already at or
+    /// below `max_size`, and trims it when over. Pins the `>` boundary.
+    #[test]
+    fn truncate_boundary_behavior() {
+        let mut a = ParetoArchive::<u32>::new(space_min2());
+        // Three mutually non-dominated members.
+        a.insert(cand(1, vec![1.0, 3.0]));
+        a.insert(cand(2, vec![2.0, 2.0]));
+        a.insert(cand(3, vec![3.0, 1.0]));
+        assert_eq!(a.members().len(), 3);
+        // max_size == len → no-op (kills `>` → `>=`).
+        a.truncate(3);
+        assert_eq!(a.members().len(), 3);
+        // max_size > len → no-op.
+        a.truncate(10);
+        assert_eq!(a.members().len(), 3);
+        // max_size < len → trims.
+        a.truncate(2);
+        assert_eq!(a.members().len(), 2);
+    }
+
+    /// A trade-off candidate (better on one axis, worse on the other) is
+    /// neither dominated nor dominating — it must be *added* alongside the
+    /// existing member. Pins the per-axis `<` / `>` scan in both
+    /// `member_dominates_or_equals` and `candidate_dominates_member`.
+    #[test]
+    fn trade_off_candidate_is_kept_alongside() {
+        let mut a = ParetoArchive::<u32>::new(space_min2());
+        a.insert(cand(1, vec![1.0, 5.0]));
+        a.insert(cand(2, vec![5.0, 1.0])); // trade-off — must be kept
+        assert_eq!(a.members().len(), 2);
+    }
+
+    /// An equal-objectives candidate is rejected (a member dominates-or-
+    /// equals it). Pins the Equal branch — distinguishes `<=` from `<` in
+    /// `candidate_dominates_member` and the `<=` in
+    /// `member_dominates_or_equals`'s infeasible branch.
+    #[test]
+    fn equal_candidate_is_rejected() {
+        let mut a = ParetoArchive::<u32>::new(space_min2());
+        a.insert(cand(1, vec![2.0, 2.0]));
+        a.insert(cand(2, vec![2.0, 2.0])); // identical objectives → rejected
+        assert_eq!(a.members().len(), 1);
+        assert_eq!(a.members()[0].decision, 1);
+    }
+
+    /// Two infeasible candidates: the one with smaller constraint violation
+    /// wins. Pins the `<` / `<=` in the infeasible branches.
+    #[test]
+    fn infeasible_candidate_with_smaller_violation_evicts_larger() {
+        let mut a = ParetoArchive::<u32>::new(space_min2());
+        a.insert(Candidate::new(1u32, Evaluation::constrained(vec![0.0, 0.0], 1.0)));
+        // Smaller violation → dominates the existing infeasible member.
+        a.insert(Candidate::new(2u32, Evaluation::constrained(vec![9.0, 9.0], 0.5)));
+        assert_eq!(a.members().len(), 1);
+        assert_eq!(a.members()[0].decision, 2);
+    }
 }