feat(async): add run_async to every algorithm in the catalog

Async coverage was incomplete in 0.7 (only RandomSearch and
DifferentialEvolution had run_async). 0.8 closes the gap: every one
of the 33 algorithms now exposes
run_async(&problem, concurrency).await, gated on the async feature.

- Population-based algorithms fan out per-generation evaluations
  through evaluate_batch_async with concurrency-bounded
  FuturesOrdered chunks.
- Steady-state algorithms (HillClimber, SimulatedAnnealing,
  OnePlusOneEs, Paes, NelderMead) await each step sequentially;
  they accept the concurrency parameter for API uniformity.
- TabuSearch fans out the K-neighbor batch each step.
- Surrogate algorithms (BayesianOpt, Tpe) batch the initial design
  and await per-iteration acquisitions sequentially so the surrogate
  can update between picks.
- Hyperband uses a new AsyncPartialProblem trait (mirroring
  PartialProblem for multi-fidelity workloads) and a parallel
  evaluate_batch_at_budget_async helper; each Successive-Halving
  rung fans out its budgeted evaluations.

All paths preserve seeded determinism: RNG draws happen on the main
task in the same order as the sync path, and only the evaluations
are concurrent.

Adds a dedicated cookbook recipe at docs/book/src/cookbook/async.md
with a worked example (DifferentialEvolution under tokio) and
guidance on picking concurrency. Cross-references in SUMMARY.md
and cookbook.md are updated to surface the new recipe.

The follow-up docs commit reconciles the rest of the user guide
and README to describe the new feature; this commit is the bare
async surface.

src/algorithms/parallel_eval_async.rs

@@ -5,8 +5,9 @@
 
 use futures::stream::{FuturesOrdered, StreamExt};
 
-use crate::core::async_problem::AsyncProblem;
+use crate::core::async_problem::{AsyncPartialProblem, AsyncProblem};
 use crate::core::candidate::Candidate;
+use crate::core::evaluation::Evaluation;
 
 /// Evaluate every decision concurrently against `problem`, preserving
 /// input order in the returned vector. Concurrency is bounded by
@@ -56,3 +57,35 @@ where
     }
     out
 }
+
+/// Evaluate every decision at the given `budget` concurrently against a
+/// multi-fidelity `problem`, preserving input order. Hyperband's async
+/// path uses this for each Successive-Halving rung.
+pub async fn evaluate_batch_at_budget_async<P>(
+    problem: &P,
+    decisions: &[P::Decision],
+    budget: f64,
+    concurrency: usize,
+) -> Vec<Evaluation>
+where
+    P: AsyncPartialProblem,
+{
+    assert!(
+        concurrency >= 1,
+        "evaluate_batch_at_budget_async concurrency must be >= 1"
+    );
+    let mut out: Vec<Evaluation> = Vec::with_capacity(decisions.len());
+    let mut idx = 0usize;
+    while idx < decisions.len() {
+        let mut futs = FuturesOrdered::new();
+        let end = (idx + concurrency).min(decisions.len());
+        for d in &decisions[idx..end] {
+            futs.push_back(async move { problem.evaluate_at_budget_async(d, budget).await });
+        }
+        while let Some(e) = futs.next().await {
+            out.push(e);
+        }
+        idx = end;
+    }
+    out
+}