refactor(app): clarify application and binary roles

Rename the application Cargo package from irtt-cli to irtt-rs
(crates/irtt-cli -> crates/irtt-app), and hard-rename its scriptable
client binary from irtt-cli to irtt-client, now that the application
also ships a first-class server and the old name reads as an umbrella
CLI for both roles.

Give each dedicated binary (irtt-client, irtt-tui, irtt-server) its
own entry point instead of funneling through the multicall
dispatcher's argv0/subcommand logic, so a dedicated binary only links
the applet code for the role it implements and always runs that role
regardless of the name it's invoked or copied under. irtt-rs remains
the feature-selectable multicall dispatcher and is now the only binary
that inspects argv0 or subcommands.

Move future application releases to the irtt-rs/vX.Y.Z tag namespace
(regenerated via `dist generate`), and bump the application package to
0.6.0 as a breaking distribution change. Library crate versions are
unaffected.

crates/irtt-app/src/signal.rs

@@ -0,0 +1,10 @@
+use std::sync::{
+    atomic::{AtomicBool, Ordering},
+    Arc,
+};
+
+pub fn install_signal_handler(shutdown_requested: Arc<AtomicBool>) -> Result<(), ctrlc::Error> {
+    ctrlc::set_handler(move || {
+        shutdown_requested.store(true, Ordering::Relaxed);
+    })
+}