test: strengthen durable behavior coverage after review

crates/irtt-stats/src/ipdv.rs

@@ -199,10 +199,23 @@ mod tests {
             ) {
                 let mut tracker = IpdvTracker::new(Some(limit));
                 for seq in seqs {
-                    tracker.insert(ipdv_sample(seq, i128::from(seq)));
+                    let completed_before = tracker.completed_pairs.clone();
+                    let pairs = tracker.insert(ipdv_sample(seq, i128::from(seq)));
+                    let emitted: HashSet<_> = pairs.iter().map(|pair| pair.current_seq).collect();
+                    prop_assert_eq!(emitted.len(), pairs.len());
+                    prop_assert!(emitted.is_disjoint(&completed_before), "a retained pair must not be emitted twice");
                     prop_assert!(tracker.samples.len() <= limit);
                     prop_assert!(tracker.sample_order.len() <= limit);
                     prop_assert!(tracker.completed_pairs.len() <= limit);
+                    // Eviction must remove stale pair markers as well as samples;
+                    // otherwise retained state can prevent future adjacent pairs.
+                    prop_assert_eq!(tracker.sample_order.len(), tracker.samples.len());
+                    prop_assert_eq!(tracker.sample_order.iter().collect::<HashSet<_>>().len(), tracker.samples.len());
+                    prop_assert!(tracker.sample_order.iter().all(|seq| tracker.samples.contains_key(seq)));
+                    prop_assert!(tracker.completed_pairs.iter().all(|seq| {
+                        tracker.samples.contains_key(seq)
+                            && tracker.samples.contains_key(&seq.wrapping_sub(1))
+                    }), "completed pairs must retain both sequence endpoints");
                 }
             }