Move variable resolution to middleend

- Introduce AST-based visitor and resolver in middleend
- Update guard field and parser to use AstNode<Expr>
- Remove backend variable_resolution and adjust exports
- Expose middleend in the library

src/backend/environment.rs

@@ -79,6 +79,13 @@ impl<T: Clone + Debug + PartialEq> EnvironmentStack<T> {
             format!("Undefined variable '{}'", name),
         )
     }
+
+    pub fn depth(&self) -> usize {
+        self.stack.len()
+    }
+    pub fn scope_contains(&self, index: usize, name: &str) -> bool {
+        self.stack[index].contains_key(name)
+    }
 }
 
 #[cfg(test)]