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-protocol/src/lib.rs

@@ -0,0 +1,20 @@
+//! # UnShell Protocol
+//!
+//! The protocol crate owns the wire types, framing, validation helpers, and the
+//! small tree runtime used by endpoint implementations.
+
+#![no_std]
+
+pub extern crate alloc;
+#[allow(unused_extern_crates)]
+extern crate self as unshell;
+
+/// Keep the historical nested path so existing imports and proc-macro output can
+/// continue to target `unshell::protocol::...` while the implementation lives in
+/// its own crate.
+pub mod protocol;
+
+pub use protocol::*;
+
+#[cfg(test)]
+pub use unshell_macros::{Leaf, Procedure, procedures};