feat(async): AsyncProblem trait + run_async on RandomSearch and DifferentialEvolution

Adds the headline async/await capability for IO-bound evaluations
(HTTP services, RPC clients, spawned subprocesses) — the
differentiator vs pymoo / hyperopt / MOEA Framework.

No public-API breaks for synchronous users. The new surface is
gated behind a new `async` feature flag.

- core::async_problem::AsyncProblem trait (async fn evaluate_async).
- algorithms::parallel_eval_async::evaluate_batch_async helper using
  futures::stream::FuturesOrdered with concurrency-bounded chunks;
  preserves input order so seeded determinism holds when evaluations
  are themselves deterministic.
- run_async on RandomSearch and DifferentialEvolution.
- examples/async_eval.rs: simulated 20 ms remote service. concurrency=1
  → 4.2 s, concurrency=4 → 2.1 s (2× speedup).

Bumps Cargo.toml to 0.8.0; CHANGELOG entry covers the above plus a
note that 0.6.0/0.7.0 on crates.io are yanked experimentals and 0.8
picks up cleanly from 0.5.

src/prelude.rs

@@ -8,6 +8,8 @@ pub use crate::core::{
     Candidate, Direction, Evaluation, Objective, ObjectiveSpace, OptimizationResult,
     PartialProblem, Population, Problem, Rng, rng_from_seed,
 };
+#[cfg(feature = "async")]
+pub use crate::core::async_problem::AsyncProblem;
 
 pub use crate::traits::{Initializer, Optimizer, Repair, Variation};