feat(pareto): add pareto_front and best_candidate

`pareto_front` returns all candidates not dominated by any other in
input order (O(N²·M), acceptable for v1 per spec §9.3).

`best_candidate` is the single-objective "best" finder: returns None
unless there is exactly one objective; ignores infeasibles; returns None
if every candidate is infeasible (spec §9.4).

Both re-exported from the prelude.

src/prelude.rs

@@ -11,4 +11,4 @@ pub use crate::core::{
 
 pub use crate::traits::{Initializer, Optimizer, Variation};
 
-pub use crate::pareto::{Dominance, pareto_compare};
+pub use crate::pareto::{Dominance, best_candidate, pareto_compare, pareto_front};