feat: add constraint handling with feasibility-aware trial ranking

Add constraint support so that optimization problems with constraints
(e.g., "model size < 100MB") prefer feasible solutions. Convention:
constraint value <= 0.0 means feasible.

- Add constraint_values field to Trial with set_constraints/getter
- Add constraints field to CompletedTrial with is_feasible() method
- Propagate constraints through complete_trial/prune_trial
- Make best_trial() and top_trials() constraint-aware: feasible trials
  rank above infeasible, infeasible ranked by total violation

tests/integration.rs

@@ -2141,3 +2141,107 @@ fn test_into_iterator_preserves_insertion_order() {
     let ids: Vec<u64> = (&study).into_iter().map(|t| t.id).collect();
     assert_eq!(ids, vec![0, 1, 2]);
 }
+
+// =============================================================================
+// Tests: Constraint handling
+// =============================================================================
+
+#[test]
+fn test_is_feasible_all_satisfied() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+    let mut trial = study.create_trial();
+    trial.set_constraints(vec![-1.0, 0.0, -0.5]);
+    study.complete_trial(trial, 1.0);
+
+    let completed = study.best_trial().unwrap();
+    assert!(completed.is_feasible());
+}
+
+#[test]
+fn test_is_feasible_one_violated() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+    let mut trial = study.create_trial();
+    trial.set_constraints(vec![-1.0, 0.5, -0.5]);
+    study.complete_trial(trial, 1.0);
+
+    let completed = study.best_trial().unwrap();
+    assert!(!completed.is_feasible());
+}
+
+#[test]
+fn test_is_feasible_empty_constraints() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+    let trial = study.create_trial();
+    study.complete_trial(trial, 1.0);
+
+    let completed = study.best_trial().unwrap();
+    assert!(completed.is_feasible());
+}
+
+#[test]
+fn test_best_trial_prefers_feasible() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+
+    // Infeasible trial with better objective
+    let mut trial1 = study.create_trial();
+    trial1.set_constraints(vec![1.0]);
+    study.complete_trial(trial1, 0.1);
+
+    // Feasible trial with worse objective
+    let mut trial2 = study.create_trial();
+    trial2.set_constraints(vec![-1.0]);
+    study.complete_trial(trial2, 100.0);
+
+    let best = study.best_trial().unwrap();
+    assert_eq!(best.id, 1); // feasible trial wins
+    assert_eq!(best.value, 100.0);
+}
+
+#[test]
+fn test_best_trial_feasible_by_objective() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+
+    // Feasible, worse objective
+    let mut trial1 = study.create_trial();
+    trial1.set_constraints(vec![-1.0]);
+    study.complete_trial(trial1, 10.0);
+
+    // Feasible, better objective
+    let mut trial2 = study.create_trial();
+    trial2.set_constraints(vec![-0.5]);
+    study.complete_trial(trial2, 2.0);
+
+    let best = study.best_trial().unwrap();
+    assert_eq!(best.id, 1); // lower objective wins among feasible
+    assert_eq!(best.value, 2.0);
+}
+
+#[test]
+fn test_top_trials_ranks_feasible_above_infeasible() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+
+    // Infeasible, low violation
+    let mut t0 = study.create_trial();
+    t0.set_constraints(vec![0.5]);
+    study.complete_trial(t0, 1.0);
+
+    // Feasible, worst objective among feasible
+    let mut t1 = study.create_trial();
+    t1.set_constraints(vec![-1.0]);
+    study.complete_trial(t1, 50.0);
+
+    // Feasible, best objective among feasible
+    let mut t2 = study.create_trial();
+    t2.set_constraints(vec![-0.1]);
+    study.complete_trial(t2, 5.0);
+
+    // Infeasible, high violation
+    let mut t3 = study.create_trial();
+    t3.set_constraints(vec![3.0]);
+    study.complete_trial(t3, 0.5);
+
+    let top = study.top_trials(4);
+    let ids: Vec<u64> = top.iter().map(|t| t.id).collect();
+    // Feasible sorted by objective first (5.0, 50.0), then infeasible by violation (0.5, 3.0)
+    assert_eq!(ids, vec![2, 1, 0, 3]);
+}