docs: add test folder layout explanation and E2E test section to README

- Document Vitest vs Playwright test locations and purposes
- Add E2E test section with `pnpm test:e2e` command
- Clarify that `test/browser/**` uses Vitest over HTTP, not Playwright
- Update peer dependency range to include Next.js 16.0.3

Author: JannikZed

README.md

@@ -214,6 +214,12 @@ To run all tests you can use the following command:
 pnpm build && pnpm test
 ```
 
+Folder layout / runners:
+
+- **Vitest** (unit + integration) lives in `src/**/*.test.ts(x)` and `test/**`.
+- **Playwright** (E2E) lives in `tests/**` (see `playwright.config.ts`).
+- `test/browser/**` contains Vitest tests that hit a running Next.js app over HTTP. Despite the folder name, this is not Playwright and does not use Vitest browser mode.
+
 ### Unit tests
 
 To run unit tests you can use the following command:
@@ -230,6 +236,14 @@ To run integration tests you can use the following command:
 pnpm build && pnpm test:integration
 ```
 
+### E2E tests (Playwright)
+
+To run Playwright tests (`tests/**`) you can use:
+
+```bash
+pnpm test:e2e
+```
+
 The integration tests will start a Next.js server and test the caching handler. You can modify testing behavior by setting the following environment variables:
 
 - SKIP_BUILD: If set to true, the integration tests will not build the Next.js app. Therefore the nextjs app needs to be built before running the tests. Or you execute the test once without skip build and the re-execute `pnpm test:integration` with skip build set to true.