perf: time the zarr export's four phases — 5h07m with no breakdown could not be attributed (#501) - #502

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-python@a26af69be951a213d495a4c3e4e4022e16d87065, astral-sh/setup-uv@38f3f104447c67c051c4a08e39b64a148898af3a. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/