fix: remove allowCompilationErrors and pin Unity 6 xref map

allowCompilationErrors: true was masking the root cause (missing Unity
assembly references). With the glob-based csproj now providing full
Unity type resolution, compilation succeeds cleanly and the flag is
no longer needed. Removing it makes future compilation errors visible.

Pin the UnityXrefMaps URL to the Unity 6 (6000.0) version-specific map
instead of the floating "latest" root URL, matching ProjectVersion.txt.

https://claude.ai/code/session_01N4YoJdJc2JDaoAbk5Nt8vf

Author: claude

Documentation/docfx.json

@@ -12,7 +12,6 @@
             "globalNamespaceId": "Global",
             "filter": "filterConfig.yml",
             "dest": "api",
-            "allowCompilationErrors": true,
             "disableGitFeatures": true
         }
     ],
@@ -70,7 +69,7 @@
             }
         },
         "xref": [
-            "https://normanderwan.github.io/UnityXrefMaps/xrefmap.yml",
+            "https://normanderwan.github.io/UnityXrefMaps/6000.0/xrefmap.yml",
             "https://learn.microsoft.com/en-us/dotnet/.xrefmap.json"
         ],
         "dest": "../_site"