Reuse SignatureSolver across videos (cache per player JS + preprocessed player) - #137

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, swift-actions/setup-swift@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/