Ruff 0.16.1 stabilizes ISC004, which is selected by this repository's ISC rule group. I reproduced the failing hook at 0bdcc38: it reports 12 existing unparenthesized string concatenations in cpplint_unittest.py, while the same tree passes with Ruff 0.15.20. Most are intentional wrapped strings that need explicit parentheses; cpplint_unittest.py:4000-4001 appears to intend a separate empty source line and may need a comma instead.

All hosted build and test jobs pass, so this lint migration is the remaining blocker. This draft should stay unapproved until those cases are resolved and the updated hook is green.