Phase 556: record whiteboard persistence deferred decision

[ExplodingUFO]

Summary

• Records Phase 556 / GitHub Phase 556: whiteboard primitive persistence implementation decision #230 / avalonia-node-map-bck as a deferred whiteboard primitive persistence decision.
• Extends the internal GraphWhiteboardPrimitivePersistenceDecision contract with outcome and boundary coverage for GraphDocument, workspace, clipboard, screenshot, and scene export artifacts.
• Updates bilingual roadmap, Cookbook, and public API inventory docs to state there is no GraphDocument schema change, no schema version bump, no workspace persistence behavior change, and no saved whiteboard primitive state.

Verification

• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --no-restore --logger "console;verbosity=minimal" (882 passed)
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --no-restore --logger "console;verbosity=minimal" (301 passed)
• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --no-restore --filter "FullyQualifiedName~WhiteboardPrimitivePersistenceDecisionContractsTests|FullyQualifiedName~EditorClipboardAndFragmentCompatibilityTests|FullyQualifiedName~GraphEditorExportContractsTests" --logger "console;verbosity=minimal" (20 passed)
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --no-restore --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests|FullyQualifiedName~DemoCookbookDocsTests|FullyQualifiedName~DemoCookbookCatalogTests" --logger "console;verbosity=minimal" (74 passed)
• .\eng\validate-public-api-surface.ps1 -Configuration Release -Framework net9.0 (PUBLIC_API_DIFF_GATE_OK:True)
• git diff --check (exit 0; CRLF warnings only)

Stack

• Base: PR Phase 555: add whiteboard primitive eraser behavior #235 / feature/phase-555-whiteboard-eraser-behavior
• Do not merge before Phase 555 / PR Phase 555: add whiteboard primitive eraser behavior #235 and the lower stack land.
• Lowest known stack blocker remains PR Record Phase 538 eraser feasibility boundary #200 review.

Closes #230

[ExplodingUFO]

Restacked this PR onto the latest Phase 555 head after #235 was rewritten.

Evidence:

• Base: feature/phase-555-whiteboard-eraser-behavior at 8dc4dd0fbaf8613655167329e12e29e9adbc7158
• Head: 32921a0eba26046062c2d69cd0be4d8097b97a0f
• git range-diff e38fe46..1dc3aab 8dc4dd0..HEAD: the three Phase 556 commits are equivalent after restack
• Local verification:
  • dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --filter "FullyQualifiedName~WhiteboardPrimitivePersistenceDecisionContractsTests" -> 4/4 passed
  • dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests" -> 56/56 passed
  • dotnet build AsterGraph.sln -c Release --no-restore -> 0 warnings / 0 errors
  • dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --no-build -> 882/882 passed
  • dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --no-build -> 301/301 passed
  • git diff --check 8dc4dd0..HEAD -> clean
• GitHub now reports MERGEABLE / CLEAN; all branch/PR checks passed, including hygiene-proof, framework-matrix net8.0/net9.0/net10.0, linux-validation, macos-validation, contract-proof, and release-validation.