refactor(examples): split multi-concept examples into focused single-topic files

- Split pruning_and_callbacks into pruning and early_stopping
- Split advanced_features into async_parallel, journal_storage, ask_and_tell, multi_objective
- Each example now requires only its own feature flag
- Trim sampler_comparison winner logic and verbose header
- Update CI workflow and README to match new example names

examples/async_parallel.rs

@@ -0,0 +1,52 @@
+//! Async parallel optimization — evaluate multiple trials concurrently.
+//!
+//! Uses `optimize_parallel` with tokio to run several trials at once,
+//! reducing wall-clock time when the objective involves I/O or async work.
+//!
+//! Run with: `cargo run --example async_parallel --features async`
+
+use optimizer::prelude::*;
+
+#[tokio::main]
+async fn main() -> optimizer::Result<()> {
+    let study: Study<f64> = Study::minimize(TpeSampler::new());
+
+    let x = FloatParam::new(-5.0, 5.0).name("x");
+    let y = FloatParam::new(-5.0, 5.0).name("y");
+
+    let n_trials = 30;
+    let concurrency = 4;
+
+    println!("Running {n_trials} trials with {concurrency} concurrent workers...");
+
+    study
+        .optimize_parallel(n_trials, concurrency, {
+            let x = x.clone();
+            let y = y.clone();
+            move |mut trial| {
+                let x = x.clone();
+                let y = y.clone();
+                async move {
+                    let xv = x.suggest(&mut trial)?;
+                    let yv = y.suggest(&mut trial)?;
+
+                    // Simulate async I/O (e.g. calling an external service)
+                    tokio::time::sleep(std::time::Duration::from_millis(10)).await;
+
+                    let value = xv * xv + yv * yv;
+                    Ok::<_, optimizer::Error>((trial, value))
+                }
+            }
+        })
+        .await?;
+
+    let best = study.best_trial()?;
+    println!(
+        "Best: f({:.3}, {:.3}) = {:.6}",
+        best.get(&x).unwrap(),
+        best.get(&y).unwrap(),
+        best.value,
+    );
+
+    Ok(())
+}