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/pareto/mod.rs

@@ -1,5 +1,7 @@
 //! Pareto utilities: dominance, fronts, sorting, crowding, and an archive.
 
 pub mod dominance;
+pub mod front;
 
 pub use dominance::*;
+pub use front::*;