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

src/mir/builder/control_flow/joinir/routing.rs

@@ -42,6 +42,34 @@ pub(in crate::mir::builder) fn choose_pattern_kind(
     let has_continue = ast_features::detect_continue_in_body(body);
     let has_break = ast_features::detect_break_in_body(body);
 
+    // Phase 110: StepTree parity check (structure-only SSOT).
+    //
+    // This is dev-only; strict mode turns mismatch into a fail-fast.
+    if crate::config::env::joinir_dev_enabled() {
+        use crate::ast::Span;
+        use crate::mir::control_tree::StepTreeBuilderBox;
+
+        let loop_ast = ASTNode::Loop {
+            condition: Box::new(condition.clone()),
+            body: body.to_vec(),
+            span: Span::unknown(),
+        };
+
+        let tree = StepTreeBuilderBox::build_from_ast(&loop_ast);
+        if tree.features.has_break != has_break || tree.features.has_continue != has_continue {
+            let msg = format!(
+                "[choose_pattern_kind/STEPTREE_PARITY] step_tree(break={}, cont={}) != extractor(break={}, cont={})",
+                tree.features.has_break, tree.features.has_continue, has_break, has_continue
+            );
+
+            if crate::config::env::joinir_dev::strict_enabled() {
+                panic!("{}", msg);
+            } else {
+                trace::trace().dev("choose_pattern_kind/step_tree_parity", &msg);
+            }
+        }
+    }
+
     // Phase 193: Extract features using modularized extractor
     let features = ast_features::extract_features(condition, body, has_continue, has_break);