Remove the old leaf declaration path

Delete the deprecated Leaf derive path, migrate the remaining tests and example to leaf!, and add direct coverage for endpoint-only, TUI-only, and shared-host leaf declarations.

unshell-protocol/src/lib.rs

@@ -17,4 +17,4 @@ pub mod protocol;
 pub use protocol::*;
 
 #[cfg(test)]
-pub use unshell_macros::{Leaf, Procedure, procedures};
+pub use unshell_macros::{Procedure, leaf, procedures};