ci(macos): stage protocol-45 builds as client release 0.42.0

[akonwi]

Summary

The macOS workflow's "Build Apple Silicon Release staging app" step fails on every PR. It stages the app as Kit client release 0.39.0, which build_and_run.sh rejects:

Protocol 45 requires Kit client release >= 0.42.0

That guard came from dc8ea7f (fix(protocol): align stable protocol 45 compatibility), which was committed directly to main. The macOS workflow only runs on pull requests, so the stale pin first surfaced on #65.

This updates the staging step to 0.42.0: both the KIT_MACOS_CLIENT_RELEASE env var and the KitClientRelease Info.plist assertion. That matches client_release="0.42.0" in apps/macos/script/package_app.sh and the README. No app or script changes.

Testing

• Ran the script's release-version check locally: 0.42.0 passes; 0.39.0 fails with the error above.
• The workflow YAML parses.
• This PR's macOS run exercises the staging step end to end.