feat(tui): adopt an asynchronous Tokio runtime for the TUI frontend

Refactor TUI event handling and rendering to improve responsiveness and use a unified managed client worker.

crates/irtt-app/src/shared/client/mod.rs

@@ -3,6 +3,7 @@ pub mod config;
 pub mod prepare;
 pub mod session;
 pub mod targets;
+pub(crate) mod worker;
 
 pub use args::{
     parse_dscp, parse_duration, parse_length, parse_server_fill, parse_test_duration, parse_ttl,
@@ -13,7 +14,6 @@ pub use prepare::{
     prepare_managed_run, ManagedRunSetup, TargetSelection, MANAGED_EVENT_CAPACITY,
     STDIN_MAX_DESIRED_TARGETS, STDIN_MAX_LIVE_TARGET_GENERATIONS, STDIN_OUTCOME_HISTORY_LIMIT,
 };
-pub use session::is_shutdown_requested;
 pub use targets::{
     parse_stdin_target_set, parse_target, prepare_managed_targets, target_specs, GroupPacingArg,
     PreparedTarget, TargetArg, TargetSpec,