fix(managed): publish terminal outcomes for failed targets

Extend the managed group stream with authoritative per-target terminal outcomes and structured failure classifications. Final client events are published first, followed by exactly one TargetFinished outcome for success, peer close, no-test, opening/runtime failure, removal, or cancellation.

Scheduler-owned cleanup now stops the receiver and seals subscriptions on natural completion, eliminating the event-consumer/join circular wait while preserving queued drain. TargetEventSubscription now yields ManagedGroupEvent, so consumers must handle Client and TargetFinished variants.

Strict-timeout regressions cover mixed success/failure, all-open failure, runtime failure, no-test ordering, removal, cancellation, and disconnection before join.

crates/irtt-client/src/lib.rs

@@ -78,9 +78,10 @@ pub use event::{
 pub use managed::{
     CancellationToken, EventHub, EventSubscription, ManagedClient, ManagedClientGroup,
     ManagedClientGroupConfig, ManagedClientGroupSession, ManagedClientSession,
-    ManagedGroupEndReason, ManagedGroupOutcome, ManagedGroupPacing, ManagedTargetConfig,
-    ManagedTargetEndReason, ManagedTargetOutcome, SessionEndReason, SessionOutcome,
-    SubscriberConfig, SubscriberOverflow, TargetEvent, TargetEventSubscription, TargetId,
+    ManagedGroupEndReason, ManagedGroupEvent, ManagedGroupOutcome, ManagedGroupPacing,
+    ManagedTargetConfig, ManagedTargetEndReason, ManagedTargetFailure, ManagedTargetFailureKind,
+    ManagedTargetOutcome, SessionEndReason, SessionOutcome, SubscriberConfig, SubscriberOverflow,
+    TargetEvent, TargetEventSubscription, TargetId,
 };
 pub use session::{NegotiatedParams, NegotiationRestriction};
 pub use timing::ClientTimestamp;