refactor(client): correct late-reply docs and drop private counter tests

The counter tests instantiated the private TargetCounters and mirrored its
match arms, duplicating outcome-level coverage that already asserts
replies_received and late through ManagedTargetOutcome, and coupling a
private refactor to persistent test churn.

The new counter documentation also defined late replies in timeout-only
terms. A reply that arrives behind an already-received sequence is
classified late by process_echo_reply even when its own probe has not timed
out, so both counters are now described in terms of order and timeout.

crates/irtt-client/src/managed/model.rs

@@ -209,15 +209,19 @@ pub struct ManagedTargetOutcome {
     pub end_reason: ManagedTargetEndReason,
     /// Probes the underlying client reported as sent.
     pub packets_sent: u64,
-    /// On-time unique echo replies.
+    /// Unique echo replies that arrived in order and before their probe timed
+    /// out.
     ///
-    /// A unique reply that arrived after its probe timed out is counted by
-    /// `late` instead, so this is the count of replies that arrived in time.
+    /// A unique reply that arrives behind an already-received sequence, or
+    /// after its own probe timed out, is counted by `late` instead.
     pub replies_received: u64,
     /// Replies for a sequence that had already been answered.
     pub duplicates: u64,
-    /// Replies that arrived after their probe timed out, whether or not the
-    /// client could still match them to retained send state.
+    /// Late replies: those arriving behind an already-received sequence
+    /// (reordering) or after their own probe timed out.
+    ///
+    /// This counts every late reply, whether or not the client could still
+    /// match it to retained send state.
     pub late: u64,
     /// Warning events observed for this target.
     pub warning_events: u64,

crates/irtt-client/src/managed/task.rs

@@ -394,15 +394,16 @@ impl TargetState {
     }
 }
 
-#[derive(Debug, Default, Eq, PartialEq)]
+#[derive(Default)]
 struct TargetCounters {
     packets_sent: u64,
-    /// On-time unique echo replies.
+    /// Unique echo replies that arrived in order and before their probe timed
+    /// out.
     ///
-    /// A late unique reply is counted by `late` and deliberately not here, so
-    /// `replies_received` stays the count of replies that arrived within their
-    /// probe timeout. These counters are durable, unlike the lossy managed
-    /// presentation events.
+    /// A reply that arrives behind an already-received sequence, or after its
+    /// own probe timed out, is counted by `late` and deliberately not here.
+    /// These counters are durable, unlike the lossy managed presentation
+    /// events.
     replies_received: u64,
     duplicates: u64,
     late: u64,

crates/irtt-client/src/managed/tests.rs

@@ -2954,106 +2954,3 @@ fn terminal_subscription_is_closed() {
         Err(ManagedSubscribeError::Closed)
     ));
 }
-
-/// Build the client events target accounting distinguishes.
-fn accounting_events() -> Vec<ClientEvent> {
-    let remote: std::net::SocketAddr = "127.0.0.1:2112".parse().unwrap();
-    let at = ClientTimestamp {
-        mono: Instant::now(),
-        wall: SystemTime::UNIX_EPOCH,
-    };
-    let rtt = crate::RttSample {
-        raw: Duration::from_millis(1),
-        adjusted: None,
-        effective: crate::SignedDuration::from_nanos(1_000_000),
-    };
-    vec![
-        ClientEvent::EchoSent {
-            seq: 0,
-            remote,
-            scheduled_at: at.mono,
-            sent_at: at,
-            bytes: 64,
-            send_call: Duration::ZERO,
-            timer_error: Duration::ZERO,
-        },
-        ClientEvent::EchoReply {
-            seq: 0,
-            remote,
-            sent_at: at,
-            received_at: at,
-            rtt,
-            server_timing: None,
-            one_way: None,
-            received_stats: None,
-            bytes: 64,
-            packet_meta: crate::PacketMeta::default(),
-        },
-        ClientEvent::DuplicateReply {
-            seq: 0,
-            remote,
-            received_at: at,
-            bytes: 64,
-        },
-        ClientEvent::LateReply {
-            seq: 1,
-            highest_seen: 2,
-            remote,
-            sent_at: Some(at),
-            received_at: at,
-            rtt: Some(rtt),
-            server_timing: None,
-            one_way: None,
-            received_stats: None,
-            bytes: 64,
-            packet_meta: crate::PacketMeta::default(),
-        },
-        ClientEvent::EchoLoss {
-            seq: 2,
-            sent_at: at,
-            timeout_at: at.mono,
-        },
-        ClientEvent::Warning {
-            kind: crate::WarningKind::WrongToken,
-            message: "wrong token".to_owned(),
-            at,
-        },
-    ]
-}
-
-#[test]
-fn target_counters_account_for_each_observed_event_once() {
-    let mut counters = TargetCounters::default();
-    for event in accounting_events() {
-        counters.observe(&event);
-    }
-
-    // A late unique reply is counted as late, not as a received reply, and a
-    // probe send is not accounted here because the client owns packets_sent.
-    assert_eq!(counters.packets_sent, 0);
-    assert_eq!(counters.replies_received, 1);
-    assert_eq!(counters.duplicates, 1);
-    assert_eq!(counters.late, 1);
-    assert_eq!(counters.warning_events, 1);
-}
-
-#[test]
-fn target_counters_ignore_events_without_a_durable_count() {
-    let ignored: Vec<ClientEvent> = accounting_events()
-        .into_iter()
-        .filter(|event| {
-            matches!(
-                event,
-                ClientEvent::EchoSent { .. } | ClientEvent::EchoLoss { .. }
-            )
-        })
-        .collect();
-    assert_eq!(ignored.len(), 2);
-
-    let mut counters = TargetCounters::default();
-    for event in ignored {
-        counters.observe(&event);
-    }
-
-    assert_eq!(counters, TargetCounters::default());
-}