Caerus 0.7.0

Added

• Adding a repository signed with an unknown key now asks you to confirm
  the key's fingerprint; caerus-helper imports exactly that key
  (SYNC_TRUST <fingerprint>) and refuses any other.
• Full-system-upgrade shows which held packages have newer versions and are
  being held back.

Fixed

• xbps-sys builds on hosts where libclang cannot find stddef.h
  (falls back to the compiler's include directory).
• history.log is rotated to history.log.1 past 256 KiB.
• CAERUS_HELPER_PATH and the helper next to the executable are only used
  when owned by root or the current user and not group/world-writable.
• Remaining unwrap() calls on GTK downcasts in the package list and the
  kernel-purge dialog now fall back gracefully instead of panicking.
• caerus-helper rejects repository URLs that are not http(s)/ftp/file
  URLs or absolute paths.
• .github/workflows/rust.yml (a stale duplicate of ci.yml targeting the
  removed 0.5-dev branch) deleted; docs updated to say work happens on
  main.
• AppStream metadata now lists releases.

Changed

• caerus-helper's command loop is table-driven instead of 15 copies of
  the same empty-argument check.
• packaging/srcpkgs/caerus template updated to 0.7.0.