harden: NaN-safe sort + dimension-mismatch guard in VectorIndex

tests/vector_tests.rs

@@ -12,3 +12,14 @@ fn cosine_topk_ranks_nearest_first() {
     assert_eq!(hits[1].0, 2);
     assert!(hits[0].1 > hits[1].1);
 }
+
+#[test]
+fn nan_score_does_not_panic_and_dim_mismatch_returns_empty() {
+    // NaN in a stored vector must not panic the sort.
+    let vi = VectorIndex::from_vectors(vec![vec![1.0, 0.0], vec![f32::NAN, 1.0]]);
+    let hits = vi.top_k(&[1.0, 0.0], 2);          // must not panic
+    assert!(hits.iter().any(|&(i, _)| i == 0));   // the valid vector still found
+    // Dimension mismatch → empty, never silently mispaired.
+    let vi2 = VectorIndex::from_vectors(vec![vec![1.0, 0.0, 0.0]]);
+    assert!(vi2.top_k(&[1.0, 0.0], 1).is_empty());
+}