refactor(joinir): centralize balanced depth-scan policy decision

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

@@ -0,0 +1,34 @@
+//! BalancedDepthScanPolicyBox (Phase 107)
+//!
+//! Responsibility:
+//! - Provide a single SSOT entry point for the balanced depth-scan policy decision
+//!   so callers (routing/apply) don't duplicate Use/Reject/None handling.
+
+use crate::ast::ASTNode;
+
+use super::balanced_depth_scan_policy::{classify_balanced_depth_scan_array_end, BalancedDepthScanPolicyResult};
+use super::PolicyDecision;
+
+pub(crate) struct BalancedDepthScanPolicyBox;
+
+impl BalancedDepthScanPolicyBox {
+    pub(crate) fn decide(
+        condition: &ASTNode,
+        body: &[ASTNode],
+    ) -> PolicyDecision<BalancedDepthScanPolicyResult> {
+        let decision = 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);
+        }
+
+        decision
+    }
+}
+

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

@@ -34,3 +34,4 @@ pub(in crate::mir::builder) mod p5b_escape_derived_policy;
 pub(in crate::mir::builder) mod trim_policy;
 pub(in crate::mir::builder) mod loop_true_read_digits_policy;
 pub(in crate::mir::builder) mod balanced_depth_scan_policy;
+pub(in crate::mir::builder) mod balanced_depth_scan_policy_box;