feat(pareto): add crowding_distance

Computes per-point crowding distance over a single Pareto front (spec
§9.6):

- Returns Vec<f64> with the same length as the front index slice.
- Empty front → empty Vec.
- Front of length ≤ 2 → all f64::INFINITY.
- Boundary points along each objective receive INFINITY.
- Interior points get sum of normalized neighbor gaps; if max == min for
  an objective the contribution is zero.
- Operates on minimization-oriented objective values.

src/pareto/mod.rs

@@ -1,9 +1,11 @@
 //! Pareto utilities: dominance, fronts, sorting, crowding, and an archive.
 
+pub mod crowding;
 pub mod dominance;
 pub mod front;
 pub mod sort;
 
+pub use crowding::*;
 pub use dominance::*;
 pub use front::*;
 pub use sort::*;