feat(tui): add multi-target support and improved dashboard visualization

Enable simultaneous probing of multiple targets within the TUI using a new target status table and multi-series RTT graph. Refactor target resolution logic into a shared module to support consistent behavior across CLI and TUI components.

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

@@ -1,6 +1,7 @@
 pub mod args;
 pub mod config;
 pub mod session;
+pub mod targets;
 
 pub use args::{
     parse_dscp, parse_duration, parse_length, parse_server_fill, parse_test_duration, parse_ttl,
@@ -8,3 +9,7 @@ pub use args::{
 };
 pub use config::{expected_probe_count, DEFAULT_RECV_TIMEOUT};
 pub use session::{final_drain_duration, is_shutdown_requested, ClientSession, RECV_BUDGET};
+pub use targets::{
+    parse_labelled_target, resolved_managed_targets, target_specs, GroupPacingArg,
+    LabelledTargetArg, ResolvedTarget, TargetSpec,
+};