refactor: simplify lib.rs and update feature gating for cmd modules

crates/irtt-cli/src/cmd/mod.rs

@@ -1,3 +1,7 @@
+#[cfg(feature = "client")]
 pub mod client;
 #[cfg(feature = "tui")]
 pub mod tui;
+
+// Future command applets, such as a server applet, belong here behind their
+// own feature gate.