feat(metrics): add Schott spacing metric for Pareto fronts

Standard Schott spacing: for each front point compute the Manhattan
distance to its nearest neighbor on minimization-oriented objective
values; the spacing metric is the population standard deviation of
those nearest-neighbor distances.

Returns 0.0 for empty or single-point fronts (spec §14.1).

src/lib.rs

@@ -4,6 +4,7 @@
 
 pub mod algorithms;
 pub mod core;
+pub mod metrics;
 pub mod operators;
 pub mod pareto;
 pub mod prelude;