feat(algorithms): add HillClimber single-objective greedy local search

The simplest possible local search: start from one initializer-sampled
decision, repeatedly mutate it via the variation operator, and keep the
child only when it is strictly better than the current incumbent (with
the standard feasible-beats-infeasible / lower-violation tiebreaks
when relevant).

Single-objective only — panics with a clear message if the problem
exposes more than one objective. Deterministic under a seed. Returns
a population/front of size one (the current incumbent) so it slots
into the comparison harness like any other optimizer.

src/algorithms/mod.rs

@@ -1,6 +1,7 @@
 //! Built-in reference optimizers.
 
 pub mod differential_evolution;
+pub mod hill_climber;
 pub mod moead;
 pub mod nsga2;
 pub mod nsga3;
@@ -10,6 +11,7 @@ pub mod random_search;
 pub mod spea2;
 
 pub use differential_evolution::*;
+pub use hill_climber::*;
 pub use moead::*;
 pub use nsga2::*;
 pub use nsga3::*;