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/core/mod.rs

@@ -1,5 +1,7 @@
 //! Concrete data types and the `Problem` trait that the rest of the crate is built on.
 
+#[cfg(feature = "async")]
+pub mod async_problem;
 pub mod candidate;
 pub mod evaluation;
 pub mod objective;
@@ -9,6 +11,8 @@ pub mod problem;
 pub mod result;
 pub mod rng;
 
+#[cfg(feature = "async")]
+pub use async_problem::AsyncProblem;
 pub use candidate::*;
 pub use evaluation::*;
 pub use objective::*;