- ResponseEmitter also detects output waiting in an output buffer; with
  output_buffering on, stray output was otherwise silently prepended to
  the response body. The front controller discards it and returns a
  clean 500.
- Front controller tests pin output_buffering and cover both settings
- The cache key test reflects that Symfony Cache validates keys only
  through assert(); the cache README documents this
- CI runs the production php.ini explicitly, plus one job with the
  development php.ini