feat: add Study::summary() and Display impl

Add a human-readable summary method and Display trait for Study<V>
where V: Display. The summary shows optimization direction, trial
counts (with complete/pruned breakdown), best value, and best
parameters with their labels.

Also derive PartialOrd + Ord on ParamId for deterministic parameter
ordering in summary output.

tests/integration.rs

@@ -1892,3 +1892,75 @@ fn test_enqueue_counted_in_n_trials() {
     // All 5 trials count, including the 2 enqueued ones
     assert_eq!(study.n_trials(), 5);
 }
+
+// =============================================================================
+// Test: Study summary and Display
+// =============================================================================
+
+#[test]
+fn test_summary_with_completed_trials() {
+    let study: Study<f64> = Study::with_sampler(Direction::Minimize, RandomSampler::with_seed(1));
+    let x = FloatParam::new(0.0, 10.0).name("x");
+
+    study
+        .optimize(5, |trial| {
+            let val = x.suggest(trial)?;
+            Ok::<_, Error>(val * val)
+        })
+        .unwrap();
+
+    let summary = study.summary();
+    assert!(summary.contains("Minimize"));
+    assert!(summary.contains("5 trials"));
+    assert!(summary.contains("Best value:"));
+    assert!(summary.contains("x = "));
+}
+
+#[test]
+fn test_summary_no_completed_trials() {
+    let study: Study<f64> = Study::new(Direction::Maximize);
+    let summary = study.summary();
+    assert!(summary.contains("Maximize"));
+    assert!(summary.contains("0 trials"));
+    assert!(!summary.contains("Best value:"));
+}
+
+#[test]
+fn test_summary_with_pruned_trials() {
+    let study: Study<f64> = Study::with_sampler(Direction::Minimize, RandomSampler::with_seed(1));
+    let x = FloatParam::new(0.0, 10.0).name("x");
+
+    // Manually create some complete and pruned trials
+    for _ in 0..3 {
+        let mut trial = study.create_trial();
+        let val = x.suggest(&mut trial).unwrap();
+        study.complete_trial(trial, val);
+    }
+    for _ in 0..2 {
+        let mut trial = study.create_trial();
+        let _ = x.suggest(&mut trial).unwrap();
+        study.prune_trial(trial);
+    }
+
+    let summary = study.summary();
+    // Should show breakdown when there are pruned trials
+    if study.n_pruned_trials() > 0 {
+        assert!(summary.contains("complete"));
+        assert!(summary.contains("pruned"));
+    }
+}
+
+#[test]
+fn test_display_matches_summary() {
+    let study: Study<f64> = Study::with_sampler(Direction::Minimize, RandomSampler::with_seed(1));
+    let x = FloatParam::new(0.0, 10.0).name("x");
+
+    study
+        .optimize(3, |trial| {
+            let val = x.suggest(trial)?;
+            Ok::<_, Error>(val)
+        })
+        .unwrap();
+
+    assert_eq!(format!("{study}"), study.summary());
+}