rewrite(phase3): port iseled/i2c from c-RCP; fix ROADMAP.md's Phase 17 missing i2c entry - #135

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/upload-artifact@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/