Reinterop build issue in latest repo

### What happened?

I just pulled the repo, since I would like to use the latest Unity 6 merged changes.

I am following the instructions from:
https://github.com/CesiumGS/cesium-unity/blob/main/Documentation~/developer-setup.md

But I keep getting this error (around 373 similar errors):

Packages\com.cesium.unity\Source\Editor\CesiumEditorWindow.cs(2,7): error CS0246: The type or namespace name 'Reinterop' could not be found (are you missing a using directive or an assembly reference?)

Seems that "Reinterop" is not being found either way.

I have checked the .meta file to be the original, and I am not opening unity until running the Reinterop {#reinterop-guide}.

What could I be missing?

### Environment

Cesium for Unity version: Latest repo pull from today (03/Aug/2026)
Unity Editor Version: 6000.5.6f1
Operating System: Windows 11 Buisness

### Reproduction steps

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### Supporting evidence

_No response_

Start with Documentation~/developer-setup.md and the referenced Reinterop guide, then inspect Packages/com.cesium.unity/Source/Editor/CesiumEditorWindow.cs around the reported missing namespace. Reproduce the build on Windows 11 with Unity 6000.5.6f1 and determine what setup step is missing; done means the Reinterop errors no longer occur.

Written by the indexing model from the issue text.