feat: add StudyBuilder for fluent study construction

tests/integration.rs

@@ -2245,3 +2245,95 @@ fn test_top_trials_ranks_feasible_above_infeasible() {
     // 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]);
 }
+
+// =============================================================================
+// Test: StudyBuilder
+// =============================================================================
+
+#[test]
+fn test_builder_defaults() {
+    let study: Study<f64> = Study::builder().build();
+    assert_eq!(study.direction(), Direction::Minimize);
+}
+
+#[test]
+fn test_builder_maximize() {
+    let study: Study<f64> = Study::builder().maximize().build();
+    assert_eq!(study.direction(), Direction::Maximize);
+}
+
+#[test]
+fn test_builder_minimize() {
+    let study: Study<f64> = Study::builder().minimize().build();
+    assert_eq!(study.direction(), Direction::Minimize);
+}
+
+#[test]
+fn test_builder_direction() {
+    let study: Study<f64> = Study::builder().direction(Direction::Maximize).build();
+    assert_eq!(study.direction(), Direction::Maximize);
+}
+
+#[test]
+fn test_builder_with_sampler() {
+    let x = FloatParam::new(-5.0, 5.0);
+    let study: Study<f64> = Study::builder().sampler(TpeSampler::new()).build();
+
+    study
+        .optimize(10, |trial| {
+            let val = x.suggest(trial)?;
+            Ok::<_, Error>(val * val)
+        })
+        .unwrap();
+
+    assert_eq!(study.trials().len(), 10);
+}
+
+#[test]
+fn test_builder_with_pruner() {
+    use optimizer::NopPruner;
+
+    let study: Study<f64> = Study::builder().pruner(NopPruner).build();
+
+    assert_eq!(study.direction(), Direction::Minimize);
+}
+
+#[test]
+fn test_builder_chaining() {
+    let study: Study<f64> = Study::builder()
+        .maximize()
+        .sampler(RandomSampler::with_seed(42))
+        .pruner(optimizer::NopPruner)
+        .build();
+
+    assert_eq!(study.direction(), Direction::Maximize);
+}
+
+#[test]
+fn test_builder_with_custom_value_type() {
+    let study: Study<i32> = Study::builder().maximize().build();
+    assert_eq!(study.direction(), Direction::Maximize);
+}
+
+#[test]
+fn test_builder_optimizes_correctly() {
+    let x = FloatParam::new(-10.0, 10.0);
+    let study: Study<f64> = Study::builder()
+        .minimize()
+        .sampler(TpeSampler::builder().seed(42).build().unwrap())
+        .build();
+
+    study
+        .optimize(100, |trial| {
+            let val = x.suggest(trial)?;
+            Ok::<_, Error>((val - 3.0) * (val - 3.0))
+        })
+        .unwrap();
+
+    let best = study.best_trial().unwrap();
+    assert!(
+        best.value < 5.0,
+        "best value should be < 5.0, got {}",
+        best.value
+    );
+}