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/crowding.rs

@@ -160,4 +160,33 @@ mod tests {
         assert!(d[2].is_infinite());
         assert!(d[1].is_finite());
     }
+
+    /// Crowding distance pins the exact interior contribution: for a 3-point
+    /// 2-objective front, the middle point's distance is the sum over both
+    /// objectives of (next - prev) / span. With evenly-spaced points the
+    /// value is exactly 2.0 (1.0 per objective).
+    #[test]
+    fn interior_point_distance_is_pinned() {
+        let s = space_min2();
+        // Front along the line f1 + f2 = 4: (0,4), (2,2), (4,0).
+        let pop = [cand(vec![0.0, 4.0]), cand(vec![2.0, 2.0]), cand(vec![4.0, 0.0])];
+        let d = crowding_distance(&pop, &[0, 1, 2], &s);
+        // Boundary points are infinite; the middle point gets
+        // (4-0)/4 + (4-0)/4 = 2.0 (objective 0 span 4, objective 1 span 4).
+        assert!(d[0].is_infinite());
+        assert!(d[2].is_infinite());
+        assert!((d[1] - 2.0).abs() < 1e-12, "interior distance = {}", d[1]);
+    }
+
+    /// An asymmetric front pins the per-objective `(next - prev) / span`
+    /// arithmetic: catches the `-` ↔ `+`/`/` and `/` ↔ `*` mutants.
+    #[test]
+    fn asymmetric_interior_distance_is_pinned() {
+        let s = space_min2();
+        // (0,10), (1,2), (10,0): objective-0 span = 10, objective-1 span = 10.
+        let pop = [cand(vec![0.0, 10.0]), cand(vec![1.0, 2.0]), cand(vec![10.0, 0.0])];
+        let d = crowding_distance(&pop, &[0, 1, 2], &s);
+        // middle point: obj0 (10-0)/10 = 1.0; obj1 (10-0)/10 = 1.0 → 2.0.
+        assert!((d[1] - 2.0).abs() < 1e-12, "got {}", d[1]);
+    }
 }