test(fuzz): add cargo-fuzz harness for Pareto and operator hot paths

fuzz/fuzz_targets/spacing.rs

@@ -0,0 +1,42 @@
+#![no_main]
+//! Fuzz the `spacing` metric for non-negativity.
+
+use arbitrary::Arbitrary;
+use libfuzzer_sys::fuzz_target;
+
+use heuropt::core::candidate::Candidate;
+use heuropt::core::evaluation::Evaluation;
+use heuropt::core::objective::{Objective, ObjectiveSpace};
+use heuropt::metrics::spacing::spacing;
+
+#[derive(Arbitrary, Debug)]
+struct Input {
+    points: Vec<(f64, f64)>,
+}
+
+fuzz_target!(|input: Input| {
+    if input.points.len() > 64 {
+        return;
+    }
+    // Bound magnitudes so distance computations don't overflow to
+    // inf-inf=NaN — `spacing` is documented to operate on values produced
+    // by `as_minimization` of problem evaluations, not arbitrary f64s.
+    if input
+        .points
+        .iter()
+        .any(|&(a, b)| !a.is_finite() || !b.is_finite() || a.abs() > 1e150 || b.abs() > 1e150)
+    {
+        return;
+    }
+    let space = ObjectiveSpace::new(vec![Objective::minimize("f1"), Objective::minimize("f2")]);
+    let pop: Vec<Candidate<()>> = input
+        .points
+        .iter()
+        .map(|&(a, b)| Candidate::new((), Evaluation::new(vec![a, b])))
+        .collect();
+    let s = spacing(&pop, &space);
+    // Spacing can overflow to +∞ when point coordinates straddle ±f64::MAX.
+    // Contract is non-negative + non-NaN.
+    assert!(s >= 0.0, "spacing negative: {s}");
+    assert!(!s.is_nan(), "spacing is NaN");
+});