test(nelder_mead,nsga2,nsga3,one_plus_one_es,particle_swarm): pin selection/geometry helpers

Phase 1 tests:
- nelder_mead: compare / better feasibility-first + direction.
- nsga2: binary_tournament prefers lower rank, then higher crowding
  distance at equal rank (statistical majority over 200 seeds).
- nsga3: solve_intercepts on axis-aligned extremes / singular / empty;
  associate picks the closest reference direction with correct
  perpendicular distance.
- one_plus_one_es: worse_than across feasibility + direction + equal.
- particle_swarm: best_index min/max/tie/single-element.

src/algorithms/particle_swarm.rs

@@ -418,4 +418,33 @@ mod tests {
         let mut opt = make_optimizer(0);
         let _ = opt.run(&SchafferN1);
     }
+
+    // ---- Mutation-test pinned helpers --------------------------------------
+
+    use crate::core::objective::Direction;
+
+    #[test]
+    fn best_index_minimize_picks_smallest() {
+        let v = [3.0, 1.0, 4.0, 1.5];
+        assert_eq!(best_index(&v, Direction::Minimize), 1);
+    }
+
+    #[test]
+    fn best_index_maximize_picks_largest() {
+        let v = [3.0, 1.0, 4.0, 1.5];
+        assert_eq!(best_index(&v, Direction::Maximize), 2);
+    }
+
+    #[test]
+    fn best_index_keeps_first_on_tie() {
+        // Strict comparison → the earliest index of a tied extreme wins.
+        let v = [1.0, 1.0, 1.0];
+        assert_eq!(best_index(&v, Direction::Minimize), 0);
+        assert_eq!(best_index(&v, Direction::Maximize), 0);
+    }
+
+    #[test]
+    fn best_index_single_element() {
+        assert_eq!(best_index(&[42.0], Direction::Minimize), 0);
+    }
 }