refactor(workspace): enforce layered crate boundaries

crates/aether-runtime/base/src/shutdown.rs

@@ -0,0 +1,15 @@
+#[cfg(unix)]
+pub async fn wait_for_shutdown_signal() -> Result<(), std::io::Error> {
+    use tokio::signal::unix::{signal, SignalKind};
+
+    let mut terminate = signal(SignalKind::terminate())?;
+    tokio::select! {
+        _ = tokio::signal::ctrl_c() => Ok(()),
+        _ = terminate.recv() => Ok(()),
+    }
+}
+
+#[cfg(not(unix))]
+pub async fn wait_for_shutdown_signal() -> Result<(), std::io::Error> {
+    tokio::signal::ctrl_c().await
+}