feat(pareto): add ParetoArchive

A concrete archive (not a trait — spec §13). On insert it discards the
new candidate if any existing member dominates it, then removes existing
members the new candidate dominates. `truncate` does simple
tail-truncation in v1; the doc note flags that crowding-aware
truncation is a future improvement.

src/prelude.rs

@@ -12,6 +12,6 @@ pub use crate::core::{
 pub use crate::traits::{Initializer, Optimizer, Variation};
 
 pub use crate::pareto::{
-    Dominance, best_candidate, crowding_distance, non_dominated_sort, pareto_compare,
-    pareto_front,
+    Dominance, ParetoArchive, best_candidate, crowding_distance, non_dominated_sort,
+    pareto_compare, pareto_front,
 };