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/top_trials.rs

@@ -0,0 +1,76 @@
+use optimizer::{Direction, Study};
+
+#[test]
+fn test_top_trials_minimize() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+
+    // Manually complete trials with known values
+    for &val in &[5.0, 1.0, 3.0, 2.0, 4.0] {
+        let trial = study.create_trial();
+        study.complete_trial(trial, val);
+    }
+
+    let top3 = study.top_trials(3);
+    assert_eq!(top3.len(), 3);
+    assert_eq!(top3[0].value, 1.0);
+    assert_eq!(top3[1].value, 2.0);
+    assert_eq!(top3[2].value, 3.0);
+}
+
+#[test]
+fn test_top_trials_maximize() {
+    let study: Study<f64> = Study::new(Direction::Maximize);
+
+    for &val in &[5.0, 1.0, 3.0, 2.0, 4.0] {
+        let trial = study.create_trial();
+        study.complete_trial(trial, val);
+    }
+
+    let top3 = study.top_trials(3);
+    assert_eq!(top3.len(), 3);
+    assert_eq!(top3[0].value, 5.0);
+    assert_eq!(top3[1].value, 4.0);
+    assert_eq!(top3[2].value, 3.0);
+}
+
+#[test]
+fn test_top_trials_n_greater_than_total() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+
+    for &val in &[3.0, 1.0] {
+        let trial = study.create_trial();
+        study.complete_trial(trial, val);
+    }
+
+    let top = study.top_trials(10);
+    assert_eq!(top.len(), 2);
+    assert_eq!(top[0].value, 1.0);
+    assert_eq!(top[1].value, 3.0);
+}
+
+#[test]
+fn test_top_trials_empty() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+
+    let top = study.top_trials(5);
+    assert!(top.is_empty());
+}
+
+#[test]
+fn test_top_trials_excludes_pruned() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+
+    // Complete some trials
+    for &val in &[5.0, 1.0, 3.0] {
+        let trial = study.create_trial();
+        study.complete_trial(trial, val);
+    }
+
+    // Prune a trial (it gets a default value of 0.0 but should be excluded)
+    let trial = study.create_trial();
+    study.prune_trial(trial);
+
+    let top = study.top_trials(5);
+    assert_eq!(top.len(), 3, "pruned trial should be excluded");
+    assert_eq!(top[0].value, 1.0);
+}