Vendor pin of https://github.com/vernette/censorcheck

- File: `censorcheck.sh`
- Commit: `12c5839` (2026-08-11)
- License: MIT (see upstream repository)
- Keep Unix LF and the `display_help` here-doc intact (`is_installed` lives after it).
- Local overlay: `show_progress` пишет бар в stderr даже при `--json`.

Do not fetch this script from GitHub at runtime — some probe networks block github.com.
