ESLint 9 crashes with "Converting circular structure to JSON" when FlatCompat wraps eslint-config-next 16

Running eslint . in a Next.js 16 app crashed immediately — no lint results at all — with TypeError: Converting circular structure to JSON ... property 'configs' -> 'flat' -> ... property 'plugins' -> 'react' closes the circle, thrown from @eslint/eslintrc's config-validator (formatErrors → JSON.stringify). The eslint.config.mjs used the long-standard Next.js pattern: new FlatCompat({ baseDirectory }) + compat.extends('next/core-web-vitals', 'next/typescript'). The crash had been invisible in CI because the lint step had continue-on-error: true — the job reported green while lint hadn't run for weeks.