Prepare Release

Prepare the Cesium for Unreal release prep changes.

Inputs:

• New Cesium for Unreal version: <X.Y.Z>
• Release date: determine the closest first working day of the current or next month unless I override it.

Procedure:

1. Determine the release date as the closest first working day of the current or next month, unless I explicitly override it.
2. Find the previous release tag and review all changes since that release.
3. Check every change against CHANGES.md and identify anything missing from the changelog.
4. If anything is missing, propose the missing changelog entries before making any edits.
5. Set the pending CHANGES.md section header to the new release version and release date.
6. Update CHANGES.md so the pending release section fully represents the changes since the last release, and explicitly add the cesium-native update note.
7. Run the release-prep script directly after the changelog edits.
8. Use the script for CesiumForUnreal.uplugin, package.json, and updating extern/cesium-native by checking out main, pulling the latest changes, and staging the parent repo's submodule reference.
9. Review the resulting diff before committing anything.