fabric: tensor parallelism across CUDA processes (bootstrap, NCCL, mailbox and RoCE collectives, plan link, fail-fast) (DeepSeek-V4.1 stack 3/14) - #636

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@11d5960a326750d5838078e36cf38b85af677262, actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/