docs(serialization): freeze workspace contract

[ExplodingUFO]

Summary

• publish EN/ZH serialization contract docs and link them from host integration
• lock schema 3/4 compatibility, unknown-field tolerance, and rejection guidance in serialization tests
• add workspace load seam coverage and docs regression coverage for the frozen contract

Verification

• dotnet test tests/AsterGraph.Serialization.Tests/AsterGraph.Serialization.Tests.csproj --filter "FullyQualifiedName~SerializationCompatibilityTests" -nologo -v minimal
• dotnet test tests/AsterGraph.Editor.Tests/AsterGraph.Editor.Tests.csproj --filter "FullyQualifiedName~GraphEditorServiceSeamsTests" -nologo -v minimal
• dotnet test tests/AsterGraph.Demo.Tests/AsterGraph.Demo.Tests.csproj --filter "FullyQualifiedName~SerializationContractDocsTests" -nologo -v minimal
• dotnet build AsterGraph.sln -nologo -v minimal
• pwsh -File eng/ci.ps1 -Lane contract -Framework all -Configuration Release