Create a Next.js regression test

Add the smallest test that exercises the bug through the correct public behavior. Keep product code unchanged.

Preconditions

• Require a validated reproduction and reported-version-versus-canary result.
• Work in a current vercel/next.js source checkout with dependencies and test prerequisites available.
• If the behavior cannot be expressed reliably in the upstream test harness, report the blocker instead of adding a weak assertion.

Workflow