ci: upgrade to Python 3.13, opt in to Node.js 24 runner

- python-version: "3.12" → "3.13" in both ci.yml and pages.yml - FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true added to both workflows so actions/checkout@v4 and actions/setup-python@v5 run on Node 24 before it becomes the forced default on June 2 2026 - Rename stale step name "Generate Tour de Force" → "Generate demo pages"