Evaluate a render assertion helper for tests

## Summary

Evaluate and, if repeated boilerplate is demonstrated, add a small test helper for asserting rendered output.

`TestStore` covers state transitions and command effects, while `TestDriver` covers the production kernel. View assertions currently require tests to construct a ratatui `TestBackend`, render a frame, and inspect its buffer manually.

This should become public API only if examples or real applications show repeated setup that a stable helper can remove. A one-off convenience wrapper is not enough justification for another testing contract.

## Acceptance criteria

- [ ] Collect at least two representative call sites that repeat the same render setup.
- [ ] Decide whether a documentation recipe is sufficient or a helper materially improves the tests.
- [ ] If a helper is justified, keep it additive and thin over ratatui's `TestBackend` and buffer types.
- [ ] Cover terminal dimensions, repeated renders, and deterministic buffer assertions.
- [ ] Document whether the helper targets `Application`, `Program`, or both.