docs(contract): lock compatibility deprecation policy

[ExplodingUFO]

Summary

• clarify compatibility and deprecation policy in English and Chinese extension contracts
• keep hosted UI helper as a published supported route while making three-band deprecation policy explicit
• mark the interface BeginConnection shim obsolete and lock the compatibility/canonical split in session tests

Verification

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