refactor(stats): retain exact finite timing samples

`SampleMode::Exact` retained exact samples for eight of the eleven public
timing metrics. Send call, timer error, and server processing always built
their `TimeMetric` with retention disabled, so a finite run reported no
median for them even though they are the same public `TimeStats` type and
appear in the same summary table with a Median column.

Derive one `retain_exact` decision from the sample mode in `CoreStats::new`
and apply it to every timing metric, so exact mode means the same thing for
all of them. Running-only mode is unchanged and still retains none of them.

The retention estimator counted eight exact sample streams per measurable
reply. Rename it to the eleven exact samples an ordinary successful probe
can retain and document the wider optional-measurement upper bound, which
raises the finite estimate on this target from 668 to 764 bytes per probe.
A test counts the metrics a fully measured probe actually retains and
compares it against that constant, so retention and its estimate cannot
drift apart unnoticed.

No new median-capability type was needed: `TimeStats::median_ns` already
distinguishes a retained median from an absent one, and an optional
measurement the server never supplied still produces no samples at all.

crates/irtt-cli/src/cmd/client/summary.rs

@@ -218,6 +218,97 @@ mod tests {
         assert!(!output.contains("medians unavailable"));
     }
 
+    /// Returns the Median cell of the row with this label.
+    fn median_cell(output: &str, label: &str) -> String {
+        let row = output
+            .lines()
+            .find(|line| line.trim_start().starts_with(label))
+            .unwrap_or_else(|| panic!("summary should contain a {label:?} row:\n{output}"));
+        // Metric, Count, Min, Mean, Median, ...
+        // Counting from the right keeps multi-word labels out of the way.
+        row.split_whitespace()
+            .rev()
+            .nth(2)
+            .unwrap_or_else(|| panic!("row {row:?} should have a Median cell"))
+            .to_owned()
+    }
+
+    /// Collects probes whose send-call, timer-error, and server-processing
+    /// values each have a median distinct from their min, mean, and max, so
+    /// the rendered cell can only come from a real median calculation.
+    fn spread_finite_summary(config: StatsConfig) -> Snapshot {
+        let mut collector = StatsCollector::new(config);
+        for (seq, (send_call_us, timer_error_us, processing_us)) in
+            [(10_u64, 2_u64, 100_u64), (40, 12, 700), (20, 4, 300)]
+                .into_iter()
+                .enumerate()
+        {
+            let seq = seq as u32;
+            let offset = Duration::from_secs(1) + Duration::from_millis(u64::from(seq));
+            let sent_at = test_timestamp(offset);
+            let received_at = test_timestamp(offset + Duration::from_micros(1500));
+
+            collector.process(&ClientEvent::EchoSent {
+                seq,
+                remote: test_remote(),
+                scheduled_at: sent_at.mono,
+                sent_at,
+                bytes: 64,
+                send_call: Duration::from_micros(send_call_us),
+                timer_error: Duration::from_micros(timer_error_us),
+            });
+            collector.process(&ClientEvent::EchoReply {
+                seq,
+                remote: test_remote(),
+                sent_at,
+                received_at,
+                rtt: RttSample {
+                    raw: Duration::from_micros(1500),
+                    adjusted: None,
+                    effective: SignedDuration::from_nanos(1_500_000),
+                },
+                server_timing: Some(ServerTiming {
+                    receive_wall_ns: None,
+                    receive_mono_ns: None,
+                    send_wall_ns: None,
+                    send_mono_ns: None,
+                    midpoint_wall_ns: None,
+                    midpoint_mono_ns: None,
+                    processing: Some(Duration::from_micros(processing_us)),
+                }),
+                one_way: None,
+                received_stats: None,
+                bytes: 64,
+                packet_meta: PacketMeta::default(),
+            });
+        }
+        collector.snapshot()
+    }
+
+    #[test]
+    fn finite_summary_shows_medians_for_send_call_timer_error_and_processing() {
+        let output = format_summary(&spread_finite_summary(StatsConfig::finite()));
+
+        // Medians of 10/20/40, 2/4/12, and 100/300/700 µs. None of these
+        // equals the row's min, mean, or max.
+        assert_eq!(median_cell(&output, "send call"), "20.0µs");
+        assert_eq!(median_cell(&output, "timer error"), "4.0µs");
+        assert_eq!(median_cell(&output, "server processing"), "300.0µs");
+    }
+
+    #[test]
+    fn continuous_summary_still_shows_no_medians_for_those_rows() {
+        let output = format_summary(&spread_finite_summary(StatsConfig::continuous()));
+
+        for label in ["send call", "timer error", "server processing"] {
+            assert_eq!(
+                median_cell(&output, label),
+                "-",
+                "continuous mode retains no exact samples for {label}"
+            );
+        }
+    }
+
     #[test]
     fn verbose_summary_includes_raw_and_adjusted_rtt_rows() {
         let mut collector = StatsCollector::new(StatsConfig::finite());