feat(operators): add RealBounds initializer and GaussianMutation

`RealBounds` (Initializer<Vec<f64>>) samples each variable uniformly in
its inclusive (lo, hi) range; panics if any bound has lo > hi
(spec §11.1).

`GaussianMutation` (Variation<Vec<f64>>) clones the first parent and
adds Normal(0, sigma) noise to every element; panics on sigma <= 0.0;
does not enforce bounds in v1 (spec §11.2).

src/lib.rs

@@ -3,6 +3,7 @@
 //! full design.
 
 pub mod core;
+pub mod operators;
 pub mod pareto;
 pub mod prelude;
 pub mod traits;