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

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

src/study.rs

@@ -1790,6 +1790,31 @@ where
     }
 }
 
+impl<V> Study<V>
+where
+    V: PartialOrd + Clone,
+{
+    /// Returns an iterator over all completed trials.
+    ///
+    /// This clones the internal trial list, so it is suitable for
+    /// analysis and iteration but not for hot paths.
+    pub fn iter(&self) -> std::vec::IntoIter<CompletedTrial<V>> {
+        self.trials().into_iter()
+    }
+}
+
+impl<V> IntoIterator for &Study<V>
+where
+    V: PartialOrd + Clone,
+{
+    type Item = CompletedTrial<V>;
+    type IntoIter = std::vec::IntoIter<CompletedTrial<V>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
 impl<V> fmt::Display for Study<V>
 where
     V: PartialOrd + Clone + fmt::Display,

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]);
+}