docs(irtt-stats): Add documentation comments to public-facing types and fields

Add docstrings to clarify the purpose of statistics collection types and describe the units and behaviors of metric fields.

crates/irtt-stats/src/loss.rs

@@ -1,15 +1,29 @@
 use crate::PacketCounts;
 
 #[derive(Debug, Clone, Copy, PartialEq)]
+/// Packet loss, duplicate, and late-packet statistics.
+///
+/// Percentage fields are percentages from `0.0` to `100.0` in normal cases.
+/// Directional upstream and downstream loss can be negative when
+/// server-reported counts exceed local expectations.
 pub struct LossStats {
+    /// Locally inferred lost packets.
     pub lost_packets: u64,
+    /// Loss not attributed to a direction.
     pub unknown_loss_packets: u64,
+    /// Server-assisted upstream loss estimate, when server counts are available.
     pub upstream_loss_packets: Option<i128>,
+    /// Server-assisted downstream loss estimate, when server counts are available.
     pub downstream_loss_packets: Option<i128>,
+    /// Total packet loss percentage.
     pub packet_loss_percent: f64,
+    /// Server-assisted upstream loss percentage.
     pub upstream_loss_percent: f64,
+    /// Server-assisted downstream loss percentage.
     pub downstream_loss_percent: f64,
+    /// Duplicate reply percentage.
     pub duplicate_percent: f64,
+    /// Late reply packet percentage.
     pub late_packets_percent: f64,
 }