feat(parserbox): add shallow recursion guard to parse_program2 for Stage-B

lang/src/compiler/parser/parser_box.hako

@@ -235,6 +235,18 @@ box ParserBox {
 
   // === Top-level program parser ===
   parse_program2(src) {
+    // Shallow recursion guard for Stage‑B / selfhost callers:
+    // prevent accidental self-recursion in ParserBox.parse_program2.
+    {
+      local depth = env.get("HAKO_STAGEB_PARSER_DEPTH")
+      if depth != null && ("" + depth) != "0" {
+        print("[stageb/recursion] ParserBox.parse_program2 recursion detected")
+        // Fail-fast: return an empty Program(JSON) so caller can detect failure.
+        return "{\"version\":0,\"kind\":\"Program\",\"body\":[]}"
+      }
+      env.set("HAKO_STAGEB_PARSER_DEPTH", "1")
+    }
+
     local trace = me.trace_enabled()
 
     // Inline skip_ws to avoid VM bug: method-with-loop called from within loop
@@ -355,6 +367,7 @@ box ParserBox {
     if trace == 1 {
       print("[parser/trace:program2] done pos=" + ("" + i) + " len=" + ("" + n))
     }
+    env.set("HAKO_STAGEB_PARSER_DEPTH", "0")
     return "{\"version\":0,\"kind\":\"Program\",\"body\":" + body + "}"
   }
 }