Simulator stack: HostSim (graph sim) + HostSIL (real firmware in the loop) + ngspice backend + review hardening - #45

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, actions/upload-artifact@v4, ilammy/msvc-dev-cmd@v1.13.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/