Split protocol and leaf surfaces into crates

Move the protocol runtime into unshell-protocol and remote shell leaf code into unshell-leaves so endpoint and TUI roles can compile independently without circular dependencies.

unshell-leaves/src/remote_shell/tui.rs

@@ -0,0 +1,43 @@
+//! Placeholder client-side TUI surface for the remote shell leaf.
+//!
+//! The first application-layer consumer will be a CLI and later a full GUI. This
+//! stub keeps the leaf-specific interpretation point in place without forcing a
+//! rendering-library decision yet.
+
+use std::string::String;
+use std::vec::Vec;
+
+use unshell::Leaf;
+use unshell::protocol::DataMessage;
+
+use crate::{LeafTui, TuiError};
+
+/// Stub TUI surface for the remote shell leaf.
+#[derive(Default, Leaf)]
+#[leaf(leaf_name = "remote_shell")]
+pub struct RemoteShellTui {
+    transcript: Vec<u8>,
+}
+
+impl RemoteShellTui {
+    /// Returns a short explanation of the current stub status.
+    pub fn status_line(&self) -> &'static str {
+        "remote shell TUI stub: rendering is placeholder-only for now"
+    }
+}
+
+impl LeafTui for RemoteShellTui {
+    fn leaf_name(&self) -> String {
+        Self::protocol_leaf_name()
+    }
+
+    fn handle_data(&mut self, message: &DataMessage) -> Result<(), TuiError> {
+        self.transcript.extend_from_slice(&message.data);
+        Ok(())
+    }
+
+    fn render(&self) -> String {
+        let body = String::from_utf8_lossy(&self.transcript);
+        format!("{}\n\n{}", self.status_line(), body)
+    }
+}