feat(joinir): route balanced depth-scan via Pattern2 policy

src/mir/builder/control_flow/joinir/patterns/pattern2_inputs_facts_box.rs

@@ -16,6 +16,7 @@ use crate::mir::join_ir::lowering::debug_output_box::DebugOutputBox;
 use crate::mir::join_ir::lowering::join_value_space::JoinValueSpace;
 use crate::mir::join_ir::lowering::loop_body_local_env::LoopBodyLocalEnv;
 use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
+use crate::mir::join_ir::lowering::loop_update_analyzer::UpdateExpr;
 use crate::mir::ValueId;
 
 use crate::mir::loop_pattern_detection::function_scope_capture::CapturedEnv;
@@ -78,6 +79,15 @@ pub(in crate::mir::builder) struct Pattern2Inputs {
     /// Phase 94: BodyLocalDerived recipe for P5b "ch" reassignment + escape counter.
     pub body_local_derived_recipe:
         Option<crate::mir::join_ir::lowering::common::body_local_derived_emitter::BodyLocalDerivedRecipe>,
+    /// Phase 107: Balanced depth-scan (find_balanced_*) derived recipe.
+    pub balanced_depth_scan_recipe:
+        Option<crate::mir::join_ir::lowering::common::balanced_depth_scan_emitter::BalancedDepthScanRecipe>,
+    /// Phase 107: Carrier updates override (policy SSOT).
+    pub carrier_updates_override: Option<std::collections::BTreeMap<String, UpdateExpr>>,
+    /// Phase 107: Post-loop early return plan for return-in-loop normalization.
+    pub post_loop_early_return: Option<
+        crate::mir::builder::control_flow::joinir::patterns::policies::balanced_depth_scan_policy::PostLoopEarlyReturnPlan,
+    >,
 }
 
 pub(crate) struct Pattern2InputsFactsBox;

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

@@ -11,8 +11,37 @@ pub(crate) struct ApplyPolicyStepBox;
 
 impl ApplyPolicyStepBox {
     pub(crate) fn apply(condition: &ASTNode, body: &[ASTNode], facts: Pattern2Facts) -> Result<Pattern2Inputs, String> {
+        use super::super::policies::balanced_depth_scan_policy;
+        use super::super::policies::PolicyDecision;
         use crate::mir::join_ir::lowering::common::body_local_slot::ReadOnlyBodyLocalSlotBox;
 
+        // 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)
+        {
+            return Ok(Pattern2Inputs {
+                loop_var_name: facts.loop_var_name,
+                loop_var_id: facts.loop_var_id,
+                carrier_info: facts.carrier_info,
+                scope: facts.scope,
+                captured_env: facts.captured_env,
+                join_value_space: facts.join_value_space,
+                env: facts.env,
+                condition_bindings: facts.condition_bindings,
+                body_local_env: facts.body_local_env,
+                allowed_body_locals_for_conditions: result.allowed_body_locals_for_conditions,
+                read_only_body_local_slot: None,
+                break_condition_node: result.break_condition_node,
+                is_loop_true_read_digits: false,
+                condition_only_recipe: None,
+                body_local_derived_recipe: None,
+                balanced_depth_scan_recipe: Some(result.derived_recipe),
+                carrier_updates_override: Some(result.carrier_updates_override),
+                post_loop_early_return: Some(result.post_loop_early_return),
+            });
+        }
+
         let break_routing = Pattern2BreakConditionPolicyRouterBox::route(condition, body)?;
 
         let read_only_body_local_slot = if break_routing.allowed_body_locals_for_conditions.is_empty() {
@@ -40,7 +69,9 @@ impl ApplyPolicyStepBox {
             is_loop_true_read_digits: break_routing.is_loop_true_read_digits,
             condition_only_recipe: None,
             body_local_derived_recipe: None,
+            balanced_depth_scan_recipe: None,
+            carrier_updates_override: None,
+            post_loop_early_return: None,
         })
     }
 }
-

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

@@ -83,8 +83,11 @@ impl PromoteStepBox {
         }
 
         use crate::mir::join_ir::lowering::loop_update_analyzer::LoopUpdateAnalyzer;
-        let carrier_updates =
+        let carrier_updates = if let Some(override_map) = inputs.carrier_updates_override.take() {
-            LoopUpdateAnalyzer::analyze_carrier_updates(analysis_body, &inputs.carrier_info.carriers);
+            override_map
+        } else {
+            LoopUpdateAnalyzer::analyze_carrier_updates(analysis_body, &inputs.carrier_info.carriers)
+        };
         Pattern2DebugLog::new(verbose).log(
             "updates",
             format!("Phase 176-3: Analyzed {} carrier updates", carrier_updates.len()),

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

@@ -17,8 +17,20 @@ pub(in crate::mir::builder) fn choose_pattern_kind(
     body: &[ASTNode],
 ) -> crate::mir::loop_pattern_detection::LoopPatternKind {
     use crate::mir::builder::control_flow::joinir::patterns::ast_feature_extractor as ast_features;
+    use crate::mir::builder::control_flow::joinir::patterns::policies::balanced_depth_scan_policy::classify_balanced_depth_scan_array_end;
+    use crate::mir::builder::control_flow::joinir::patterns::policies::PolicyDecision;
     use crate::mir::loop_pattern_detection;
 
+    // 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(_)
+    ) {
+        return loop_pattern_detection::LoopPatternKind::Pattern2Break;
+    }
+
     // 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);