fix(joinir): wire balanced depth-scan policy through Pattern2

src/mir/builder/control_flow/joinir/patterns/pattern2_steps/apply_policy_step_box.rs

@@ -17,9 +17,19 @@ impl ApplyPolicyStepBox {
 
         // Phase 107: balanced depth-scan (return-in-loop) policy.
         // This route provides its own break-cond + derived recipe + post-loop early return plan.
-        if let PolicyDecision::Use(result) =
-            balanced_depth_scan_policy::classify_balanced_depth_scan_array_end(condition, body)
-        {
+        let decision = balanced_depth_scan_policy::classify_balanced_depth_scan_array_end(condition, body);
+        if crate::config::env::joinir_dev_enabled() {
+            use crate::mir::builder::control_flow::joinir::trace;
+            let summary = match &decision {
+                PolicyDecision::Use(_) => "Use".to_string(),
+                PolicyDecision::Reject(reason) => format!("Reject: {}", reason),
+                PolicyDecision::None => "None".to_string(),
+            };
+            trace::trace().dev("phase107/balanced_depth_scan_policy", &summary);
+        }
+
+        match decision {
+            PolicyDecision::Use(result) => {
                 return Ok(Pattern2Inputs {
                     loop_var_name: facts.loop_var_name,
                     loop_var_id: facts.loop_var_id,
@@ -41,6 +51,11 @@ impl ApplyPolicyStepBox {
                     post_loop_early_return: Some(result.post_loop_early_return),
                 });
             }
+            PolicyDecision::Reject(reason) => {
+                return Err(reason);
+            }
+            PolicyDecision::None => {}
+        }
 
         let break_routing = Pattern2BreakConditionPolicyRouterBox::route(condition, body)?;
 

src/mir/builder/control_flow/joinir/patterns/pattern2_steps/promote_step_box.rs

@@ -166,7 +166,11 @@ impl PromoteStepBox {
             .collect();
 
         if cond_scope.has_loop_body_local() {
-            if !inputs.is_loop_true_read_digits {
+            // Phase 107: balanced depth-scan policy already provides an allow-list + derived recipe.
+            // Do not re-run Pattern2 body-local promotion/slot heuristics here (they assume break-guard shapes).
+            if inputs.balanced_depth_scan_recipe.is_some() {
+                // no-op: LoopBodyLocalInitLowerer + BalancedDepthScanEmitter will populate the env.
+            } else if !inputs.is_loop_true_read_digits {
                 match classify_for_pattern2(
                     builder,
                     &inputs.loop_var_name,

src/mir/builder/control_flow/joinir/patterns/policies/balanced_depth_scan_policy.rs

@@ -476,6 +476,7 @@ fn eq_int(left: ASTNode, n: i64) -> ASTNode {
 mod tests {
     use super::*;
     use crate::ast::Span;
+    use crate::parser::NyashParser;
 
     fn span() -> Span {
         Span::unknown()
@@ -520,6 +521,46 @@ mod tests {
         }
     }
 
+    fn find_first_loop<'a>(node: &'a ASTNode) -> Option<(&'a ASTNode, &'a [ASTNode])> {
+        match node {
+            ASTNode::Loop { condition, body, .. } => Some((condition.as_ref(), body.as_slice())),
+            ASTNode::Program { statements, .. } => statements.iter().find_map(find_first_loop),
+            ASTNode::BoxDeclaration {
+                methods,
+                constructors,
+                static_init,
+                ..
+            } => {
+                for v in methods.values() {
+                    if let Some(found) = find_first_loop(v) {
+                        return Some(found);
+                    }
+                }
+                for v in constructors.values() {
+                    if let Some(found) = find_first_loop(v) {
+                        return Some(found);
+                    }
+                }
+                if let Some(init) = static_init {
+                    if let Some(found) = init.iter().find_map(find_first_loop) {
+                        return Some(found);
+                    }
+                }
+                None
+            }
+            ASTNode::FunctionDeclaration { body, .. } => body.iter().find_map(find_first_loop),
+            ASTNode::If {
+                then_body,
+                else_body,
+                ..
+            } => then_body
+                .iter()
+                .find_map(find_first_loop)
+                .or_else(|| else_body.as_ref()?.iter().find_map(find_first_loop)),
+            _ => None,
+        }
+    }
+
     #[test]
     fn detects_balanced_array_end_min_shape() {
         // loop(i < n) {
@@ -584,4 +625,29 @@ mod tests {
         assert!(result.carrier_updates_override.contains_key("i"));
         assert!(result.carrier_updates_override.contains_key("depth"));
     }
+
+    #[test]
+    fn detects_balanced_array_end_min_shape_from_parser_ast() {
+        let src = r#"
+static box Main {
+  f(s, idx) {
+    local n = s.length()
+    if s.substring(idx, idx+1) != "[" { return -1 }
+    local depth = 0
+    local i = idx
+    loop (i < n) {
+      local ch = s.substring(i, i+1)
+      if ch == "[" { depth = depth + 1 }
+      if ch == "]" { depth = depth - 1  if depth == 0 { return i } }
+      i = i + 1
+    }
+    return -1
+  }
+}
+"#;
+        let ast = NyashParser::parse_from_string(src).expect("parse ok");
+        let (condition, body) = find_first_loop(&ast).expect("find loop");
+        let decision = classify_balanced_depth_scan_array_end(condition, body);
+        assert!(matches!(decision, PolicyDecision::Use(_)), "got {:?}", decision);
+    }
 }

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

@@ -24,12 +24,19 @@ pub(in crate::mir::builder) fn choose_pattern_kind(
     // Phase 107: Route balanced depth-scan (return-in-loop) to Pattern2 via policy.
     //
     // This keeps Pattern routing structural: no by-name dispatch, no silent fallback.
-    if matches!(
-        classify_balanced_depth_scan_array_end(condition, body),
-        PolicyDecision::Use(_)
-    ) {
+    match classify_balanced_depth_scan_array_end(condition, body) {
+        PolicyDecision::Use(_) => {
             return loop_pattern_detection::LoopPatternKind::Pattern2Break;
         }
+        PolicyDecision::Reject(_reason) => {
+            // In strict mode, treat "close-but-unsupported" as a fail-fast
+            // Pattern2 route so the policy can surface the precise contract violation.
+            if crate::config::env::joinir_dev::strict_enabled() {
+                return loop_pattern_detection::LoopPatternKind::Pattern2Break;
+            }
+        }
+        PolicyDecision::None => {}
+    }
 
     // Phase 193: Use AST Feature Extractor Box for break/continue detection
     let has_continue = ast_features::detect_continue_in_body(body);