refactor(cli): simplify feature flags and make stats mandatory for clients

Remove the optional "stats" feature by merging it into the "client" and "tui" features to reduce conditional compilation complexity. Replace the "client-runtime" internal feature with direct dependency sets.

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

@@ -1,12 +1,12 @@
 use std::sync::atomic::{AtomicBool, Ordering};
 
-#[cfg(feature = "client-runtime")]
+#[cfg(any(feature = "client", feature = "tui"))]
 use irtt_client::managed::{ManagedEvent, ManagedEventSubscription, ManagedEventTryRecvError};
 
-#[cfg(feature = "client-runtime")]
+#[cfg(any(feature = "client", feature = "tui"))]
 pub(crate) const MANAGED_EVENT_WORK_BUDGET: usize = 128;
 
-#[cfg(feature = "client-runtime")]
+#[cfg(any(feature = "client", feature = "tui"))]
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub(crate) enum ManagedDrainState {
     Empty,
@@ -16,7 +16,7 @@ pub(crate) enum ManagedDrainState {
 
 /// Drain a bounded amount of lossy presentation work before returning to the
 /// frontend control loop.
-#[cfg(feature = "client-runtime")]
+#[cfg(any(feature = "client", feature = "tui"))]
 pub(crate) fn drain_managed_events<E>(
     events: &mut ManagedEventSubscription,
     dropped_events: &mut u64,