feat(traits): add Initializer, Variation, and Optimizer traits

The three operator-level traits the algorithms consume, plus the single
trait users implement to add a new optimizer (`Optimizer<P>`). All take
`&mut Rng` directly rather than being generic over the RNG (spec §7.8).

src/traits/initializer.rs

@@ -0,0 +1,12 @@
+//! Trait for sampling initial decisions.
+
+use crate::core::rng::Rng;
+
+/// Generates initial decisions for a population.
+///
+/// Implementations should return exactly `size` decisions; if that is
+/// impossible, panic with a clear message in v1.
+pub trait Initializer<D> {
+    /// Produce `size` initial decisions using the supplied RNG.
+    fn initialize(&mut self, size: usize, rng: &mut Rng) -> Vec<D>;
+}