feat(events): semantic events for registered named graphs, with a graph filter - #784

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. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

The ledgers name a revision that is not this code; CI re-derives them automatically (re-run-conformance). This is not a conformance regression.