docs: update tests/README and GUIDE__Development with current test patterns

tests/README.md:
- Add shared helpers table (_helpers/ files and purpose)
- Update "Writing / modifying tests" to reference setup_badge_test_page;
  badge tests are now the canonical template
- Add "Hard-Won Lessons — Badge Print / IDB Tests" section covering:
  - __version guard: why ae_defaults.ts must include __version: 1 or
    store_versions.ts silently wipes ae_loc and loses trusted_access
  - IDB inject-then-reload pattern: why reload is required (liveQuery
    won't fire on raw IDB writes that bypass Dexie's notification system)
- Fix pre-existing lint warnings: bare URLs → code spans, trailing newline

GUIDE__Development.md:
- Add Required Check #5: run Playwright integration tests for auth/store
  or badge print changes; badge tests are the canonical template
- Add tests/README.md to Key Documentation table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

documentation/GUIDE__Development.md

@@ -11,6 +11,11 @@
 2. **Type Safety:** Ensure interfaces in `src/lib/types/ae_types.ts` match backend schemas.
 3. **Reactivity Check:** Verify Svelte 5 runes (`$state`, `$derived`) are not creating race conditions with Dexie `liveQuery`.
 4. **Build Check:** For major changes, run `npm run build:staging` to ensure no SSR or build-time failures.
+5. **Integration Tests:** For changes to badge print, event layouts, or auth/store logic, run the relevant Playwright test file(s):
+   ```bash
+   npx playwright test tests/event_badge_render.test.ts tests/event_badge_attendee_workflow.test.ts
+   ```
+   Run the full suite with `npm run test:integration`. The badge tests (`event_badge_*.test.ts`) are the canonical integration test template.
 
 ## 2. Commit Policy
 - **Atomic Commits:** One component or one logic fix per commit. Do not batch unrelated changes.
@@ -46,6 +51,7 @@ You are not working in a vacuum. Coordinate with the Backend Agent via MCP tools
 | `documentation/AE__Architecture.md` | System architecture overview |
 | `documentation/AE__Naming_Conventions.md` | Naming rules |
 | `documentation/PROJECT__AE_Events_Launcher_Native_integration.md` | Electron/Launcher reference |
+| `tests/README.md` | Playwright test guide — shared helpers, hard-won lessons, demo IDs |
 
 ## 6. URL Parameters