ci: extract Unity 6 managed DLLs from game-ci Docker image

Unity managed DLLs are required for Roslyn to resolve UnityEngine types
during DocFX metadata extraction. Rather than committing binaries, the
workflow pulls the public game-ci editor image (no license required) and
copies the full UnityEngine/ directory (~60-80 modules) into lib/.

A cache keyed on the Unity version (6000.0.73f1) stores the extracted
DLLs (~50-150 MB) so Docker is only pulled on the first run or when the
Unity version changes. Subsequent runs skip the extract step entirely.

To update Unity version: bump the version in ProjectVersion.txt,
docfx.json xref URL, DocFxForUnity.csproj comment, and the cache key +
Docker tag here.

https://claude.ai/code/session_01N4YoJdJc2JDaoAbk5Nt8vf

Author: claude

.github/workflows/documentation.yml

@@ -26,6 +26,20 @@ jobs:
       - name: Install DocFX
         run: dotnet tool install -g docfx --version 2.77.0
 
+      - name: Cache Unity managed DLLs
+        id: cache-unity-dlls
+        uses: actions/cache@v4
+        with:
+          path: lib/UnityEngine
+          key: unity-managed-dlls-6000.0.73f1-v1
+
+      - name: Extract Unity managed DLLs from game-ci image
+        if: steps.cache-unity-dlls.outputs.cache-hit != 'true'
+        run: |
+          docker create --name unity-editor unityci/editor:ubuntu-6000.0.73f1-base-3
+          docker cp unity-editor:/opt/unity/Editor/Data/Managed/UnityEngine lib/
+          docker rm unity-editor
+
       - name: Use README.md as index.md
         run: cp README.md Documentation/index.md