grammar: grammar-constrained decoding on the Zig side over xgrammar 0.2.8's C++ core (DeepSeek-V4.1 stack 5/14) - #638

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