The dry-run mode hardcoded SYSTEM_CLEAN=false, which meant
the entire system section was silently skipped during preview.
Users had no way to see what /Library/Caches, /private/var/log,
diagnostics, or other sudo-required paths would be cleaned.
Now dry-run checks for a cached sudo session (sudo -n) and
includes the system preview when available. When sudo isn't
cached, a hint tells the user how to get the full preview
without running the whole script as root.
- reorganize perform_cleanup sections and naming
- merge macOS + sandbox caches into clean_app_caches
- move recent items and mail downloads into user essentials
- update core/user tests for renamed internal helpers
