Replace GitHub API artifact discovery with a static base URL, verify the
downloaded package by sha256, and record the applied checksum so the
wrapper can report when mling is already up to date.

Add `mling update` to fetch the latest GitHub Actions artifact for the
current platform, stage it as a tarball in the data directory, and apply
it on the next wrapper invocation. The wrapper extracts the staged
package over the installation directory while skipping the running
executable, then removes the staged file.

Add `mling` wrapper binary that delegates to `mingling-cli`, enabling
separate binary names while maintaining Cargo conventions.