fix(managed): bound completed target retention

Retain only the 256 most recent target outcomes in the final group snapshot while tracking total, successful, failed, and discarded outcome counts. Publish each authoritative TargetFinished event before inserting into the bounded history.

Update CLI/TUI exit decisions to use aggregate counts and add a dynamic churn regression proving every outcome remains observable while join retention stays bounded.

crates/irtt-client/src/lib.rs

@@ -82,6 +82,7 @@ pub use managed::{
     ManagedGroupPacing, ManagedTargetConfig, ManagedTargetEndReason, ManagedTargetFailure,
     ManagedTargetFailureKind, ManagedTargetOutcome, SessionEndReason, SessionOutcome,
     SubscriberConfig, SubscriberOverflow, TargetEvent, TargetEventSubscription, TargetId,
+    MANAGED_GROUP_OUTCOME_HISTORY_LIMIT,
 };
 pub use session::{NegotiatedParams, NegotiationRestriction};
 pub use timing::ClientTimestamp;