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/metrics/mod.rs

@@ -0,0 +1,5 @@
+//! Quality metrics for Pareto fronts.
+
+pub mod spacing;
+
+pub use spacing::*;