Phase 549: whiteboard persistence decision gate

[ExplodingUFO]

Summary

• Add an internal GraphWhiteboardPrimitivePersistenceDecision policy contract for Phase 549.
• Record the current decision as a separate annotation surface excluded from the current GraphDocument schema.
• Update EN/ZH React Flow parity roadmap docs and tests with GitHub Phase 549: whiteboard primitive persistence decision implementation gate #221 / avalonia-node-map-3l6 while leaving Phase 550 TBD.

Scope Boundaries

• No GraphDocument schema change.
• No schema version bump.
• No workspace persistence behavior change.
• No public drawing API, pointer coordinator, toolbar, eraser, screenshot manifest, or Cookbook visual proof work.
• Stacked after PR Phase 548: whiteboard primitive renderer adapter skeleton #220; do not merge before Phase 548 and the earlier stack land.

Verification

• RED first: focused Phase 549 tests initially failed on missing persistence decision types.
• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --filter "WhiteboardPrimitivePersistenceDecisionContractsTests|WhiteboardPrimitiveRendererAdapterContractsTests|SerializationCompatibilityTests" -v:minimal (30 passed)
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests" -v:minimal (46 passed)
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release -v:minimal (288 passed)
• dotnet build AsterGraph.sln -c Release -v:minimal (0 warnings, 0 errors)
• git diff --check (exit 0; LF-to-CRLF notices only)

Refs #221