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/managed/mod.rs

@@ -6,8 +6,9 @@ mod runner;
 pub use cancellation::CancellationToken;
 pub use group::{
     ManagedClientGroup, ManagedClientGroupConfig, ManagedClientGroupSession, ManagedGroupEndReason,
-    ManagedGroupOutcome, ManagedGroupPacing, ManagedTargetConfig, ManagedTargetEndReason,
-    ManagedTargetOutcome, TargetEvent, TargetEventSubscription, TargetId,
+    ManagedGroupEvent, ManagedGroupOutcome, ManagedGroupPacing, ManagedTargetConfig,
+    ManagedTargetEndReason, ManagedTargetFailure, ManagedTargetFailureKind, ManagedTargetOutcome,
+    TargetEvent, TargetEventSubscription, TargetId,
 };
 pub use hub::{EventHub, EventSubscription, SubscriberConfig, SubscriberOverflow};
 pub use runner::{ManagedClient, ManagedClientSession, SessionEndReason, SessionOutcome};