ci: drop node20 workflow shims

[resoltico]

Summary

• replace the Node 20-based publication artifact pin with the Node 24-capable artifact action used elsewhere
• upgrade the Codecov action pin to the Node 24-capable release and remove the forced JavaScript runtime override
• add an architecture-contract guard so release workflows cannot drift back to Node 20 compatibility shims

Testing

• npx --yes @devcontainers/cli exec --workspace-folder /Users/erst/Tools/FTLLexEngine bash -lc 'cd /workspaces/ftllexengine && export FTLLEXENGINE_DEVCONTAINER=1 UV_LINK_MODE=copy UV_PROJECT_ENVIRONMENT=.venv-devcontainer-3.13 && uv run --python 3.13 pytest tests/test_architecture_contract.py::test_release_workflows_do_not_depend_on_node20_compatibility_shims -q'
• npx --yes @devcontainers/cli exec --workspace-folder /Users/erst/Tools/FTLLexEngine ./check.sh