docs(stats): clarify server receive-window semantics

An audit of the server-reported receive window found no defect in the
statistics pipeline: the bitmap is decoded raw, stored raw, never
interpreted, and never feeds the directional loss estimates, which use
the cumulative server receive count.

The public rustdoc was silent on all of that, leaving consumers of
PacketCounts free to read a window of 0x1 as a 63-packet loss burst.
Document the field's contract, name the count as the only server input
to LossStats, and cover the window-only case with a regression test.

crates/irtt-stats/tests/stats.rs

@@ -2,7 +2,9 @@ mod common;
 
 use std::time::{Duration, Instant, UNIX_EPOCH};
 
-use common::{adjusted_reply, sent, ts, unadjusted_late_reply, unadjusted_reply};
+use common::{
+    adjusted_reply, reply_with_received_stats, sent, ts, unadjusted_late_reply, unadjusted_reply,
+};
 use irtt_client::{
     ClientEvent, ClientTimestamp, OneWayDelaySample, PacketMeta, RttSample, SignedDuration,
 };
@@ -378,6 +380,32 @@ fn empty_and_all_lost_edges_are_defined() {
     assert_eq!(all_lost.loss.packet_loss_percent, 100.0);
 }
 
+#[test]
+fn receive_window_is_stored_raw_and_never_drives_directional_loss() {
+    // Window-only negotiation: the server reports a bitmap but no receive count.
+    let mut collector = StatsCollector::new(StatsConfig::finite());
+    collector.process(&sent(0, ts(0)));
+    collector.process(&reply_with_received_stats(0, 10, None, Some(0x1)));
+
+    // `0x1` represents no earlier history. It must be preserved verbatim and
+    // must not be read as 63 lost packets or stand in for a receive count.
+    let snapshot = collector.snapshot();
+    assert_eq!(snapshot.packets.server_received_window, Some(0x1));
+    assert_eq!(snapshot.packets.server_packets_received, None);
+    assert_eq!(snapshot.loss.upstream_loss_packets, None);
+    assert_eq!(snapshot.loss.downstream_loss_packets, None);
+    assert_eq!(snapshot.loss.upstream_loss_percent, 0.0);
+    assert_eq!(snapshot.loss.downstream_loss_percent, 0.0);
+
+    // An advancing bitmap is stored verbatim across the full 64-bit range.
+    collector.process(&sent(1, ts(10)));
+    collector.process(&reply_with_received_stats(1, 10, None, Some(u64::MAX)));
+    assert_eq!(
+        collector.snapshot().packets.server_received_window,
+        Some(u64::MAX)
+    );
+}
+
 #[test]
 fn directional_loss_uses_server_received_count_when_available() {
     let mut collector = StatsCollector::new(StatsConfig::finite());