docs(contract): inventory canonical stability surfaces

[ExplodingUFO]

Summary

• make the canonical SDK stability inventory explicit in extension contracts
• clarify the adapter capability matrix is layered on the canonical contract rather than defining a second API surface
• extend release-surface regression coverage for the new stability inventory contract

Verification

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