fix: guard unified BoxCall recursion and document Stage-B stack overflow status

src/mir/builder.rs

@@ -188,6 +188,10 @@ pub struct MirBuilder {
     /// infinite recursion (emit_unified_call → emit_box_or_plugin_call →
     /// emit_unified_call …) can occur when routing decisions disagree.
     pub(super) in_unified_boxcall_fallback: bool,
+
+    /// Recursion depth counter for debugging stack overflow
+    /// Tracks the depth of build_expression calls to detect infinite loops
+    pub(super) recursion_depth: usize,
 }
 
 impl MirBuilder {
@@ -242,6 +246,7 @@ impl MirBuilder {
             schedule_mat_map: HashMap::new(),
 
             in_unified_boxcall_fallback: false,
+            recursion_depth: 0,
         }
     }
 
@@ -371,7 +376,21 @@ impl MirBuilder {
     /// Build an expression and return its value ID
     pub(super) fn build_expression(&mut self, ast: ASTNode) -> Result<ValueId, String> {
         // Delegated to exprs.rs to keep this file lean
-        self.build_expression_impl(ast)
+        // Debug: Track recursion depth to detect infinite loops
+        const MAX_RECURSION_DEPTH: usize = 200;
+        self.recursion_depth += 1;
+        if self.recursion_depth > MAX_RECURSION_DEPTH {
+            eprintln!("\n[FATAL] ============================================");
+            eprintln!("[FATAL] Recursion depth exceeded {} in build_expression", MAX_RECURSION_DEPTH);
+            eprintln!("[FATAL] Current depth: {}", self.recursion_depth);
+            eprintln!("[FATAL] AST node type: {:?}", std::mem::discriminant(&ast));
+            eprintln!("[FATAL] ============================================\n");
+            return Err(format!("Recursion depth exceeded: {} (possible infinite loop)", self.recursion_depth));
+        }
+
+        let result = self.build_expression_impl(ast);
+        self.recursion_depth -= 1;
+        result
     }