fix: resolve Ubuntu 26 setup fallback

[alvins82]

Summary

• resolve setup assets using distro ID/codename first
• fall back through distro ID and ID_LIKE when a codename-specific directory is unavailable
• suppress expected lookup misses during fallback

Root cause

Ubuntu 26 reports the resolute codename, but no ubuntu/resolute setup directory exists. The previous resolver emitted pushd errors instead of selecting the generic Ubuntu setup.

Impact

UserService setup runs cleanly on Ubuntu 26 while retaining existing distro-specific precedence.

Validation

• bash -n on the updated script
• synthetic Ubuntu 26 resolver check
• native Ubuntu 26.04 arm64 fresh install and post-reboot service health