test: POSIX-normalise paths in link_priority_structural_test (fixes 4 Windows-only failures) - #247

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.)