feat(parserstmt): add shallow recursion guard to ParserStmtBox.parse

lang/src/compiler/parser/stmt/parser_stmt_box.hako

@@ -9,6 +9,17 @@ using lang.compiler.parser.stmt.parser_exception_box
 
 static box ParserStmtBox {
   parse(src, i, ctx) {
+    // Shallow recursion guard: prevent ParserStmtBox.parse from recursively
+    // re-entering itself on the same VM stack（Stage‑B/selfhost 安全弁）。
+    {
+      local depth = env.get("HAKO_STAGEB_STMT_DEPTH")
+      if depth != null && ("" + depth) != "0" {
+        print("[stageb/recursion] ParserStmtBox.parse recursion detected")
+        // Fail-fast: return empty stmt and keep position unchanged
+        return ""
+      }
+      env.set("HAKO_STAGEB_STMT_DEPTH", "1")
+    }
     local j = ctx.skip_ws(src, i)
     local stmt_start = j
 
@@ -182,6 +193,8 @@ static box ParserStmtBox {
       if j < src.length() { j = j + 1 } else { j = src.length() }
     }
     ctx.gpos_set(j)
+    // Clear depth guard before returning
+    env.set("HAKO_STAGEB_STMT_DEPTH", "0")
     return "{\"type\":\"Expr\",\"expr\":" + e + "}"
   }