replace self_update crate with lightweight custom updater

self_update expected tar.gz archives with full Rust target triples
but our release workflow uploads raw binaries with shortened names.
This caused 'xmaster update --check' to report stale versions.

New updater:
- Hits GitHub API directly for latest release
- Maps compile-time target to our asset naming convention
- Downloads raw binary and atomic-renames into place
- Includes downgrade protection (semver comparison)
- Removes self_update dependency (saves ~2MB, 12 transitive deps)

Author: longevityboris