refactor(tests): split integration.rs into focused subfolders

- Delete 20 duplicate tests already covered by parameter_tests.rs
- Move 11 pure Trial unit tests into src/trial.rs
- Split remaining 84 integration tests into tests/study/ (9 modules)
- Group sampler tests into tests/sampler/ (7 modules)
- Group pruner tests into tests/pruner/ (2 modules)

tests/study/constraints.rs

@@ -0,0 +1,101 @@
+use optimizer::{Direction, Study};
+
+#[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]);
+}