Add PHP test + QA harness (Docker testbed, PHPStan/Rector, CI) - #1

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, arduino/setup-task@v2, crazy-max/ghaction-github-runtime@v3, docker/setup-buildx-action@v3, dorny/test-reporter@v1. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/