Add rollback command for reverting to a previous release

Added

• xpilot rollback command: install an earlier GitHub Release — without --version it rolls back to the most recent release strictly older than the running version, and with --version X.Y.Z it installs the specified version. Installation reuses the --update asset-selection logic (wheel first, then sdist tarball, falling back to building from the tagged source when neither is present). To support this, xpilot/updater.py gains fetch_releases, pick_previous_release, find_release_by_version, and perform_rollback; the "pick asset + invoke pip install" step shared by --update and rollback is extracted into the internal helper _install_release. Adds corresponding unit tests in tests/test_updater.py.

Full Changelog: v0.2.0...v0.3.0