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

fuzz/fuzz_targets/hypervolume_2d.rs

@@ -0,0 +1,47 @@
+#![no_main]
+//! Fuzz `hypervolume_2d` for non-negativity and reference-point handling.
+
+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::hypervolume::hypervolume_2d;
+
+#[derive(Arbitrary, Debug)]
+struct Input {
+    points: Vec<(f64, f64)>,
+    ref_point: (f64, f64),
+}
+
+fuzz_target!(|input: Input| {
+    if input.points.len() > 64 {
+        return;
+    }
+    // Non-finite floats are permitted by Evaluation, but HV is undefined
+    // there — restrict to finite for this property.
+    if !input.ref_point.0.is_finite() || !input.ref_point.1.is_finite() {
+        return;
+    }
+    if input
+        .points
+        .iter()
+        .any(|&(a, b)| !a.is_finite() || !b.is_finite())
+    {
+        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 hv = hypervolume_2d(&pop, &space, [input.ref_point.0, input.ref_point.1]);
+    // HV can be +∞ when the dominated rectangle area overflows f64 (e.g. a
+    // ref point at f64::MAX with deeply negative front coords). The
+    // contracted invariants are non-negativity and non-NaN.
+    assert!(hv >= 0.0, "HV negative: {hv}");
+    assert!(!hv.is_nan(), "HV is NaN");
+});