feat: implement IntoIterator for &Study and add iter() method

Enables idiomatic `for trial in &study` iteration over completed trials.

tests/integration.rs

@@ -2097,3 +2097,47 @@ fn test_retries_with_zero_max_retries_same_as_optimize() {
     assert_eq!(call_count.get(), 5);
     assert_eq!(study.n_trials(), 5);
 }
+
+// =============================================================================
+// Tests: IntoIterator for &Study
+// =============================================================================
+
+#[test]
+fn test_into_iterator_iterates_all_trials() {
+    let study: Study<f64> = Study::with_sampler(Direction::Minimize, RandomSampler::with_seed(42));
+    let x_param = FloatParam::new(0.0, 10.0);
+
+    for _ in 0..5 {
+        let mut trial = study.create_trial();
+        let x = x_param.suggest(&mut trial).unwrap();
+        study.complete_trial(trial, x * x);
+    }
+
+    let mut count = 0;
+    for trial in &study {
+        assert_eq!(trial.state, optimizer::TrialState::Complete);
+        count += 1;
+    }
+    assert_eq!(count, 5);
+}
+
+#[test]
+fn test_into_iterator_empty_study() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+
+    let count = (&study).into_iter().count();
+    assert_eq!(count, 0);
+}
+
+#[test]
+fn test_into_iterator_preserves_insertion_order() {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+
+    for i in 0..3 {
+        let trial = study.create_trial();
+        study.complete_trial(trial, f64::from(i));
+    }
+
+    let ids: Vec<u64> = (&study).into_iter().map(|t| t.id).collect();
+    assert_eq!(ids, vec![0, 1, 2]);
+}