install (root) downloads the self-contained joltc release asset for the host
platform, verifies its sha256, and drops the binary in /usr/local/bin (--dir /
--version override). Resolves the latest release via the GitHub API, clears the
macOS quarantine flag, and backs up an existing joltc. Modeled on babashka's
installer. README gets a one-line curl|bash install.
