feat(algorithms): add DifferentialEvolution (DE/rand/1/bin)

Optional v1 algorithm requested by the user (spec §12.4):

- Vec<f64> decisions only.
- Single-objective only — panics with a clear message otherwise.
- Standard DE/rand/1/bin: for each target i, sample distinct r1, r2, r3;
  mutant = x[r1] + F * (x[r2] - x[r3]); apply binomial crossover with at
  least one forced index; greedy replacement on direction-correct
  comparison.
- Bounds taken from the embedded RealBounds (mutants are clamped to the
  per-variable range so the trial vector stays feasible).
- Seed-deterministic; tests verify reproducibility, that DE improves on
  the initial random population for a sphere problem, and that
  multi-objective use panics.

src/prelude.rs

@@ -19,5 +19,6 @@ pub use crate::pareto::{
 pub use crate::operators::{BitFlipMutation, GaussianMutation, RealBounds, SwapMutation};
 
 pub use crate::algorithms::{
-    Nsga2, Nsga2Config, Paes, PaesConfig, RandomSearch, RandomSearchConfig,
+    DifferentialEvolution, DifferentialEvolutionConfig, Nsga2, Nsga2Config, Paes, PaesConfig,
+    RandomSearch, RandomSearchConfig,
 };