feat(client): implement multi-target ManagedClientGroup for shared-socket probing

Introduce a managed group runner to support concurrent probing of multiple targets over a single UDP socket with coordinated send pacing. Refactor the existing client logic into a reusable SessionRuntime to support both individual and grouped session management.

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

@@ -1,7 +1,13 @@
 mod cancellation;
+mod group;
 mod hub;
 mod runner;
 
 pub use cancellation::CancellationToken;
+pub use group::{
+    ManagedClientGroup, ManagedClientGroupConfig, ManagedClientGroupSession, ManagedGroupEndReason,
+    ManagedGroupOutcome, ManagedGroupPacing, ManagedTargetConfig, ManagedTargetEndReason,
+    ManagedTargetOutcome, TargetEvent, TargetEventSubscription, TargetId,
+};
 pub use hub::{EventHub, EventSubscription, SubscriberConfig, SubscriberOverflow};
 pub use runner::{ManagedClient, ManagedClientSession, SessionEndReason, SessionOutcome};