test: add comprehensive FlowView test harness - #251

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, shivammathur/setup-php@ec406be512d7077f68eed36e63f4d91bc006edc4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/