2026.9.28.3: the build-plugin architecture -- mcpp.core, build information, members built once, batched placement, pack -p, the library interface (#734) - #735

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/