feat(control_tree): add StepTreeContract and signature (dev-only)

src/mir/builder/control_flow/joinir/patterns/ast_feature_extractor.rs

@@ -70,6 +70,18 @@ pub(crate) fn detect_break_in_body(body: &[ASTNode]) -> bool {
     false
 }
 
+/// Detect if a loop body contains return statements
+///
+/// This is used for dev-only parity checks with structure SSOT (StepTree).
+pub(crate) fn detect_return_in_body(body: &[ASTNode]) -> bool {
+    for stmt in body {
+        if has_return_node(stmt) {
+            return true;
+        }
+    }
+    false
+}
+
 /// Extract full feature set from loop body AST
 ///
 /// This is the main entry point for feature extraction. It analyzes the loop body
@@ -273,6 +285,26 @@ fn has_break_node(node: &ASTNode) -> bool {
     }
 }
 
+/// Recursive helper to check if AST node contains return
+fn has_return_node(node: &ASTNode) -> bool {
+    match node {
+        ASTNode::Return { .. } => true,
+        ASTNode::If {
+            then_body,
+            else_body,
+            ..
+        } => {
+            then_body.iter().any(has_return_node)
+                || else_body
+                    .as_ref()
+                    .map_or(false, |e| e.iter().any(has_return_node))
+        }
+        ASTNode::Loop { body, .. } => body.iter().any(has_return_node),
+        ASTNode::ScopeBox { body, .. } => body.iter().any(has_return_node),
+        _ => false,
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

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

@@ -41,6 +41,7 @@ pub(in crate::mir::builder) fn choose_pattern_kind(
     // Phase 193: Use AST Feature Extractor Box for break/continue detection
     let has_continue = ast_features::detect_continue_in_body(body);
     let has_break = ast_features::detect_break_in_body(body);
+    let has_return = ast_features::detect_return_in_body(body);
 
     // Phase 110: StepTree parity check (structure-only SSOT).
     //
@@ -56,10 +57,18 @@ pub(in crate::mir::builder) fn choose_pattern_kind(
         };
 
         let tree = StepTreeBuilderBox::build_from_ast(&loop_ast);
-        if tree.features.has_break != has_break || tree.features.has_continue != has_continue {
+        if tree.features.has_break != has_break
+            || tree.features.has_continue != has_continue
+            || tree.features.has_return != has_return
+        {
             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
+                "[choose_pattern_kind/STEPTREE_PARITY] step_tree(break={}, cont={}, ret={}) != extractor(break={}, cont={}, ret={})",
+                tree.features.has_break,
+                tree.features.has_continue,
+                tree.features.has_return,
+                has_break,
+                has_continue,
+                has_return
             );
 
             if crate::config::env::joinir_dev::strict_enabled() {