Align serialization docs with schema version 6

[ExplodingUFO]

Summary

• Closes Align serialization contract docs with schema version 6 #260.
• Updates EN/ZH serialization contract docs from workspace/document schema version 5 to current schema version 6.
• Updates SerializationContractDocsTests so stale schema 5 wording fails.

Verification

• RED: dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter FullyQualifiedName~SerializationContractDocsTests -v minimal failed while docs still described schema 5.
• GREEN: dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter FullyQualifiedName~SerializationContractDocsTests -v minimal passed: 1/1.
• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --filter "FullyQualifiedName~SerializationCompatibilityTests" -v minimal passed: 25/25.
• git diff --check passed with LF/CRLF warnings only.

Scope guard

Docs/tests only. No serializer behavior change, no GraphDocument schema change, no schema version bump, no migration behavior change, no workspace save/load integration change, no clipboard/export behavior change, no UI work, and no React Flow parity claim expansion.