refactor(cli): centralize managed target preparation

Both client applets duplicated the step from parsed arguments to a runnable
managed setup: target specs, managed target preparation, a ClientConfig
built from an arbitrary primary target, pacing conversion, and an identical
ManagedClientConfig skeleton with its own copy of the event-capacity
constant. The primary-target step was the worst of it, since the managed
driver replaces server_addr per target, so the address the applets went out
of their way to find was immediately discarded.

shared::client::prepare now performs that step once: it validates the target
set first, then returns the validated targets, one target-free ClientConfig
template, and the pacing, with managed_config()/managed_targets() for the
driver. CommonClientArgs::to_client_config no longer takes a server address.

Applet-specific semantics are unchanged: each command keeps its own duration
default and help text, the client still accepts no target for
--list-columns, and the TUI still rejects an empty target list at parse
time.

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

@@ -1,5 +1,6 @@
 pub mod args;
 pub mod config;
+pub mod prepare;
 pub mod session;
 pub mod targets;
 
@@ -8,6 +9,7 @@ pub use args::{
     ClockArg, CommonClientArgs, ReceivedStatsArg, TimestampArg,
 };
 pub use config::{expected_probe_count, DEFAULT_RECV_TIMEOUT};
+pub use prepare::{prepare_managed_run, ManagedRunSetup, TargetSelection, MANAGED_EVENT_CAPACITY};
 pub use session::is_shutdown_requested;
 pub use targets::{
     parse_labelled_target, prepare_managed_targets, target_specs, GroupPacingArg,