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/pareto/mod.rs

@@ -1,10 +1,12 @@
 //! Pareto utilities: dominance, fronts, sorting, crowding, and an archive.
 
+pub mod archive;
 pub mod crowding;
 pub mod dominance;
 pub mod front;
 pub mod sort;
 
+pub use archive::*;
 pub use crowding::*;
 pub use dominance::*;
 pub use front::*;