Writes and runs Flutter end-to-end user-journey tests on a real device or emulator with the official integration_test package. Use when a multi-screen Flutter app needs journey smoke tests covering navigation + DI + real app startup together, when .teikk/SPEC.md declares E2E integration_test, or when invoked via /teikk-e2e. Do NOT use for unit tests, widget component tests, or golden-only widget tests.
