Add BatchDomain::batch_ka_agree_dec, a per-viewing-key batched agreement hook - #13

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, auguwu/clippy-action@1.3.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

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