radar-install

Install rdar, which provides radar.

1. Require explicit user authorization before machine changes.
2. Run cargo --version; if missing, stop with the Rust/Cargo prerequisite. Never install a toolchain implicitly.
3. If radar --version works and no update was requested, report it and stop.
4. Run cargo install --locked rdar; add --force only for an explicit update or repair.
5. Run radar --version. If it is not on PATH, report Cargo's bin directory.

Do not map repositories; use radar-bootstrap.