feat(errors): actionable fatals, log-capture for direct mutations

error_fatal gained an optional recovery-hint arg and now prints the last log lines, the full log path, and the resume pointer on every fatal; all 17 call sites carry specific hints. The silent sed/file-write mutations (locale.gen, pacman.conf, waybar battery, regdom, geoclue, paccache, udev, fstab, mkinitcpio, sudoers) now capture stderr to the log and warn on failure, and the run summary tells the reader how to find each error's output. No new shellcheck findings; 10 unit suites green.

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