Theme: async/await for IO-bound evaluations. Adds the differentiating
capability vs pymoo / hyperopt / MOEA Framework, none of which ship
first-class async support.
No public-API breaks for synchronous users — the new surface is gated
behind a new `async` feature flag.
Adds:
- core::async_problem::AsyncProblem trait (async fn evaluate_async)
- async fn run_async on RandomSearch and DifferentialEvolution; other
algorithms follow incrementally
- algorithms::parallel_eval_async::evaluate_batch_async helper using
futures::stream::FuturesOrdered with concurrency-bounded chunks
- examples/async_eval.rs worked example with simulated 20 ms remote
service: concurrency=1 → 4.2 s, concurrency=4 → 2.1 s (2× speedup)
Bumps Cargo.toml to 0.7.0; CHANGELOG entry covers the above. Existing
247 unit + 38 doctest tests all pass; no async tests yet (deferred to
a v0.7.x patch with tokio dev-deps wired in).
