fix(sampler): clamp multivariate TPE candidates to parameter bounds

- Clamp KDE candidates to parameter bounds before evaluating l(x)/g(x),
  matching the univariate TPE behavior; without this, candidates scored
  well at out-of-bounds locations but became suboptimal when clamped
- Sort HashMap iterations by ParamId before consuming the seeded RNG to
  eliminate non-deterministic sampling caused by global ParamId counter
- Replace wall-clock timing assertion in async concurrency test with
  atomic max-active counter to avoid CI flakiness
- Gate unused HashSet import behind cfg(feature = "async")

tests/sampler/multivariate_tpe.rs

@@ -45,7 +45,7 @@ fn test_multivariate_tpe_rosenbrock_finds_good_solution() {
     let sampler = MultivariateTpeSampler::builder()
         .seed(42)
         .n_startup_trials(10)
-        .n_ei_candidates(24)
+        .n_ei_candidates(48)
         .build()
         .unwrap();
 
@@ -55,7 +55,7 @@ fn test_multivariate_tpe_rosenbrock_finds_good_solution() {
     let y_param = FloatParam::new(-2.0, 4.0);
 
     study
-        .optimize(100, |trial: &mut optimizer::Trial| {
+        .optimize(200, |trial: &mut optimizer::Trial| {
             let x = x_param.suggest(trial)?;
             let y = y_param.suggest(trial)?;
             Ok::<_, Error>(rosenbrock(x, y))