Drive a real UI through a scripted smoke path and assert observable outcomes, so a UI claim is backed by a runtime check that could have failed - backend-abstracted across Playwright, browser-use, or a computer-use harness. Use when a build needs behavioral proof it works for a real user, as the VERIFY backend for a gauntlet loop, as the band-4 runtime evidence archipelago demands, or when the user says "smoke test", "does it actually work", "click through the app", "behavioral UI test". Differentiator - the verdict is a coded assertion not a screenshot; transport-complete proves a SHA is served (liveness), this proves the UI behaves (behavior).
