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/config.rs

@@ -7,9 +7,13 @@ use super::args::CommonClientArgs;
 pub const DEFAULT_RECV_TIMEOUT: Duration = Duration::from_millis(20);
 
 impl CommonClientArgs {
-    pub fn to_client_config(&self, server: &str, duration: Duration) -> ClientConfig {
+    /// Build the shared client configuration template for a managed run.
+    ///
+    /// The template carries no server address: the managed driver replaces it
+    /// per target from that target's configuration, so deriving one here from
+    /// an arbitrary target would be dead data.
+    pub fn to_client_config(&self, duration: Duration) -> ClientConfig {
         ClientConfig {
-            server_addr: server.to_owned(),
             duration: (!duration.is_zero()).then_some(duration),
             interval: self.interval,
             length: self.length,