integration-e2e-testing

# Integration and E2E Testing Principles ## References **E2E test design**: See [references/e2e-design.md](references/e2e-design.md) for UI Spec-driven E2E test candidate selection and browser test architecture. The reference uses Playwright as the default browser harness; substitute the project's standard when different. ## Test Type Definition and Standard Budgets | Test Type | Purpose | Scope | External Deps | Standard Budget per Input Design Doc | Implementation Timing | |-----------|---------|-------|---------------|-------------------|----------------------| | Integration | Verify component interactions in-process | Partial system integration (in-process modules; for UI components, the framework's in-process renderer e.g., RTL+MSW for React/TS) | Mocked or in-process | MAX 3 | Created alongside implementation | | fixture-e2e | Verify UI behavior in a browser with deterministic fixtures | Full UI flow with mocked backend / fixture-driven state | Mocked / fixture only — no live services | MAX 3 | Created alongside the UI feature | | service-integration-e2e | Verify critical user journeys against a running local stack | Full system across services | Live local services or stubs | MAX 1-2 | Executed only in the final phase | **Lane selection (E2E only)**: - Default lane for user-facing UI journeys is **fixture-e2e** — it runs a real browser against deterministic fixtures, catches the bugs that unit/integration tests miss (button no-op, state never updates, navigation b...