- Refactor updater.mjs: extract shared utilities, remove dead alpha
channel, add `autobuild` channel via CLI arg
- Add override-updater-endpoints.mjs to switch tauri.conf.json
endpoints to updater-autobuild at build time
- Add release-update-autobuild job to autobuild workflow
- Change allowDowngrades to true so custom compareVersions handles
build-metadata versions (+autobuild.MMDD.hash) correctly
- Fix original bug: linux-x86/i686 wrote .url instead of .signature
in the .sig handler
