Discovery: Path to CoreCLR in Unity 6.8

Discovery task to map the path from Cesium for Unity's current Mono-based scripting to CoreCLR in Unity 6.8 and identify required changes. [Path to CoreCLR](https://discussions.unity.com/t/path-to-coreclr-2026-upgrade-guide/1714279#p-7279285-fast-enter-play-mode-default-for-new-projects-unity-66-18)

Start with the issue and its linked Unity discussion on the Path to CoreCLR. Map Cesium for Unity's current Mono-based scripting path and identify the repository changes required for Unity 6.8; no specific files or tests are named. Done means the migration path and required changes are documented.

Written by the indexing model from the issue text.