FCITX_INSTALL_ADDONDIR varies by distro (multiarch Ubuntu → /usr/lib/x86_64-linux-gnu/fcitx5/; Fedora → /usr/lib64/fcitx5/; Arch → /usr/lib/fcitx5/). Extract from cmake cache at build time via GITHUB_ENV instead of hardcoding. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>