test: protect durable behavior identified in review

crates/irtt-stats/tests/rolling_properties.rs

@@ -1,6 +1,6 @@
 //! Independent count and time windows must match a filtered replay through
 //! StatsCollector, while cumulative accounting retains the complete history.
-//! Generated events use non-decreasing timestamps, as required by the collector.
+//! Generated events have non-decreasing normalized window timestamps.
 
 use std::time::{Duration, Instant, UNIX_EPOCH};
 
@@ -170,7 +170,7 @@ fn build_event(op: &Op, seq: u32, clock_ms: u64, base: Instant) -> ClientEvent {
         Op::Loss => ClientEvent::EchoLoss {
             seq,
             sent_at: ts_at(base, clock_ms),
-            timeout_at: ts_at(base, clock_ms).mono,
+            timeout_at: ts_at(base, clock_ms).mono - Duration::from_millis(10),
         },
         Op::Warning => ClientEvent::Warning {
             kind: irtt_client::WarningKind::UntrackedReply,
@@ -233,6 +233,11 @@ proptest! {
         // scratch after every step.
         let mut history: Vec<(ClientEvent, u64)> = Vec::new();
 
+        // Force a loss whose clamped time evicts the initial send, while its
+        // raw pre-send deadline would retain it. Every generated case must
+        // therefore distinguish the two normalization choices.
+        let ops = [Op::Send, Op::Advance(u16::try_from(time_limit_ms + 5).unwrap()), Op::Loss, Op::Advance(10), Op::Warning]
+            .into_iter().chain(ops).collect::<Vec<_>>();
         for (idx, op) in ops.iter().enumerate() {
             if let Op::Advance(delta) = op {
                 clock_ms += u64::from(*delta);