- platform.sh: new nh_quit_instance <source> — quits the site-shared instance
  after panel login tests; skips when a harvest holds the port runlock
- app.py: login-test / login-test-src / login-discover background threads now
  recycle the instance when the test finishes (login state lives in profile,
  unaffected); manual-login flow (ensure-chromium --login) intentionally left open