feat: add Differential Evolution sampler

Population-based optimizer with mutation + crossover. Supports three
strategies (Rand1, Best1, CurrentToBest1), configurable F and CR,
and auto-sized populations. No extra dependencies needed.

src/sampler/mod.rs

@@ -3,7 +3,7 @@
 pub mod bohb;
 #[cfg(feature = "cma-es")]
 pub mod cma_es;
-pub mod de;
+pub mod differential_evolution;
 #[cfg(feature = "gp")]
 pub mod gp;
 pub mod grid;