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/early_stopping.rs

@@ -0,0 +1,43 @@
+//! Early stopping — halt an entire study once a target is reached.
+//!
+//! Use `optimize_with_callback` to inspect each completed trial and return
+//! `ControlFlow::Break(())` when the study should stop (e.g. a quality
+//! threshold is met or a time budget is exhausted).
+//!
+//! Run with: `cargo run --example early_stopping`
+
+use std::ops::ControlFlow;
+
+use optimizer::prelude::*;
+
+fn main() -> optimizer::Result<()> {
+    let study: Study<f64> = Study::new(Direction::Minimize);
+    let x = FloatParam::new(-10.0, 10.0).name("x");
+
+    let target = 0.01;
+
+    study.optimize_with_callback(
+        100, // upper bound — we expect to stop much earlier
+        |trial| {
+            let xv = x.suggest(trial)?;
+            Ok::<_, Error>((xv - 3.0).powi(2))
+        },
+        |_study, completed| {
+            if completed.value < target {
+                println!("Target {target} reached at trial #{}", completed.id);
+                return ControlFlow::Break(());
+            }
+            ControlFlow::Continue(())
+        },
+    )?;
+
+    let best = study.best_trial()?;
+    println!(
+        "Stopped after {} trials — best f({:.4}) = {:.6}",
+        study.n_trials(),
+        best.get(&x).unwrap(),
+        best.value,
+    );
+
+    Ok(())
+}