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.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "heuropt"
-version = "0.5.0"
+version = "0.8.0"
 edition = "2024"
 rust-version = "1.85"
 authors = ["Stephen Waits <steve@waits.net>"]
@@ -17,8 +17,10 @@ categories = ["algorithms", "science", "mathematics", "simulation"]
 default = []
 serde = ["dep:serde"]
 parallel = ["dep:rayon"]
+async = ["dep:futures"]
 
 [dependencies]
+futures = { version = "0.3", optional = true, default-features = false, features = ["std", "async-await"] }
 rand = "0.9"
 rand_distr = "0.5"
 rayon = { version = "1", optional = true }
@@ -27,11 +29,16 @@ serde = { version = "1", features = ["derive"], optional = true }
 [dev-dependencies]
 gungraun = "0.18"
 proptest = "1"
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "time"] }
 
 [[bench]]
 name = "hot_paths"
 harness = false
 
+[[example]]
+name = "async_eval"
+required-features = ["async"]
+
 # Tighten release codegen for the compare harness and downstream binaries
 # that build heuropt directly (i.e. when this crate is the workspace root).
 # When heuropt is used as a dependency the consumer's profile wins.