CI: split the test suite across six parallel jobs (84 min → 30 measured) - #702

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/

Deliberate short lag is fine — just make sure it's a conscious call, not a forgotten one. (This exact gap shipped edge a full day behind protocol's CRC8-length-check fix on 2026-07-30.)

openstrap_protocol is pinned at b4fde6ab5589e53154b808f8b2bbb4d01d9c15b5 but protocol's main has moved to 1ae61afc2f5088a3ae4e757af4922d541f4c4b70.