refactor(client): centralize managed target accounting

Target-local accounting was spread across the managed driver: the event
counter increments were inline in publish_client_events, the authoritative
packets_sent was copied from the client at fifteen separate call sites, and
ManagedTargetOutcome was constructed twice from the same six fields.

TargetCounters::observe now owns the per-event accounting,
TargetRuntime::sync_packets_sent/record_packets_sent own the authoritative
sent count, and TargetRuntime::outcome owns outcome construction, replacing
the free target_outcome function and the inline copy in finish_target. No
counter is touched outside TargetRuntime any more.

The replies_received semantics are unchanged and now documented on both the
private counter and the public outcome field: on-time unique echo replies
only, with late unique replies counted by late. observe matches ClientEvent
exhaustively, so a new event variant must be classified deliberately.

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

@@ -198,16 +198,28 @@ pub enum ManagedEndReason {
 }
 
 /// Durable outcome for one target generation.
+///
+/// These counters are durable: unlike [`ManagedEvent`], which is a lossy
+/// presentation stream, they account for everything the target observed.
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct ManagedTargetOutcome {
     pub target: TargetInstance,
     pub server_addr: Arc<str>,
     pub remote: Option<SocketAddr>,
     pub end_reason: ManagedTargetEndReason,
+    /// Probes the underlying client reported as sent.
     pub packets_sent: u64,
+    /// On-time unique echo replies.
+    ///
+    /// 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.
     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.
     pub late: u64,
+    /// Warning events observed for this target.
     pub warning_events: u64,
     pub cleanup_failure: Option<ManagedTargetFailure>,
 }