Add runtime API redesign scaffold

unshell-runtime/src/node/state.rs

@@ -0,0 +1,15 @@
+//! Node lifecycle state.
+
+/// Lifecycle state for a runtime node.
+#[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
+pub enum NodeState {
+    /// The node has been constructed but has not started transport activity.
+    #[default]
+    Created,
+    /// The node is accepting local work and transport events.
+    Running,
+    /// The node is draining work before shutdown.
+    Stopping,
+    /// The node has stopped and should not accept new work.
+    Stopped,
+}