Common need (assert list/result counts). Missing from the current assertion set.

Scope

• Add the have.length case in src/asserts/index.ts (positive + negative messages, and the case in the switch). Accept a number as the expected length.
• Works on a single element against el.children.length. If a collection variant is desired, leave a TODO for a follow-up.
• Add a unit test in src/tests/asserts/.
• Add a row in docs/api/assertions.md.
• Add a small list in examples/twd-test-app/src/pages/Assertions.tsx with data-testid="have-length" and a sample in assertion-examples.twd.test.ts using screenDom.getByTestId(...) + twd.should(el, "have.length", 3).

Reference

• Mirror the have.class implementation.
• Query pattern: examples/twd-test-app/src/twd-tests/screen-queries.twd.test.ts.

Acceptance criteria

• npm run test:ci is green.
• The new example test runs green in the TWD sidebar of twd-test-app.
• docs/api/assertions.md updated.
• PR includes a screenshot of the sidebar.

How to contribute

Read CONTRIBUTING.md first. Quick steps for this issue:

1. Fork, clone, create a feature branch: git checkout -b feat/have-length-assertion.
2. From the repo root, build the library and sync it to the example apps:

   npm install
   npm run build
   npm run copy:mock-sw

3. Install and run the test app:

   cd examples/twd-test-app
   npm install
   npm run dev

4. Open the app in the browser — the TWD sidebar appears on the right. Run your new tests from the sidebar and confirm they pass.
5. Run the library's unit tests from the repo root: npm run test:ci.
6. Open the PR with screenshots of the sidebar showing green tests.

Always prefer screenDom (Testing Library) for element queries. Avoid twd.get() in new code.