feat(control_tree): add StepTree builder (dev-only)

src/mir/builder/calls/lowering.rs

@@ -166,6 +166,12 @@ impl MirBuilder {
 
     /// 🎯 箱理論: Step 4 - 本体lowering
     fn lower_function_body(&mut self, body: Vec<ASTNode>) -> Result<(), String> {
+        if crate::config::env::joinir_dev_enabled() {
+            let tree = crate::mir::control_tree::StepTreeBuilderBox::build_from_block(&body);
+            crate::mir::builder::control_flow::joinir::trace::trace()
+                .dev("control_tree/step_tree", &tree.to_compact_string());
+        }
+
         eprintln!("[DEBUG/lower_function_body] body.len() = {}", body.len());
         let program_ast = function_lowering::wrap_in_program(body);
         eprintln!("[DEBUG/lower_function_body] About to call build_expression");