feat: add optional parallel feature for population-evaluation parallelism

Adds a `parallel` Cargo feature that pulls in rayon and parallelizes
the only step that's actually expensive in practice — calls to
`Problem::evaluate` — across the population. RNG-driven steps (parent
and donor selection, variation, replacement decisions) stay serial, so
seeded runs remain deterministic regardless of feature state, and the
default and `--features parallel` builds produce bit-identical
results.

Wiring:
- New `algorithms::parallel_eval::evaluate_batch` helper with two
  cfg-gated implementations (rayon's `into_par_iter` when the feature
  is on, plain `into_iter` otherwise). Both preserve input order, so
  pareto_front and crowding-distance decisions remain reproducible.
- `RandomSearch`, `Nsga2`, and `DifferentialEvolution` now route
  population/offspring evaluation through the helper. NSGA-II's main
  loop is restructured into a serial selection-and-variation phase
  followed by a parallel-friendly batch evaluation phase.
- DE's per-target loop is restructured into three phases (serial trial
  construction → batch evaluation → serial replacement). Side effect
  of the restructuring: DE is now the canonical synchronous DE/rand/1/bin
  rather than the asynchronous variant where target `i+1` sees `i`'s
  in-flight update. Synchronous is the textbook formulation, so this
  is a small correctness improvement on top of the parallelism enable.
- PAES stays serial — its main loop has a sequential dependency on the
  current candidate and would gain nothing from rayon.

Cost: algorithm impls now require `P: Sync` and `P::Decision: Send`
unconditionally so a single impl serves both feature modes. This is a
small bound tightening that any plain-data Problem already satisfies; in
return the public `Problem` trait itself stays unchanged and the
default build picks up no new dependencies.

Verified:
- `cargo test` and `cargo test --features parallel` both pass; the
  Nsga2 `deterministic_with_same_seed` test confirms reproducibility.
- `cargo run --release --example benchmarks` and the same with
  `--features parallel` produce bit-identical ZDT1 / Rastrigin
  results.

Cargo.toml

@@ -9,8 +9,10 @@ readme = "README.md"
 [features]
 default = []
 serde = ["dep:serde"]
+parallel = ["dep:rayon"]
 
 [dependencies]
 rand = "0.9"
 rand_distr = "0.5"
+rayon = { version = "1", optional = true }
 serde = { version = "1", features = ["derive"], optional = true }