Phase 557: record whiteboard annotation store contract gate

[ExplodingUFO]

Summary

• Records Phase 557 as the whiteboard annotation store contract gate in EN/ZH React Flow parity docs.
• Defines the separate annotation-store contract requirements before any saved whiteboard primitive state can exist.
• Adds a focused roadmap-doc guard test for the Phase 557 tracker IDs, boundaries, and non-implementation language.

Scope Boundaries

• No production annotation store.
• No GraphDocument schema change or schema version bump.
• No workspace persistence behavior, clipboard/export serialization, screenshot manifest, renderer, pointer, toolbar, or eraser changes.
• No saved whiteboard primitive state or full React Flow whiteboard parity claim.

Verification

• RED: Phase 557 focused docs test failed before docs update because Phase 557 was absent.
• GREEN: dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --no-restore --filter "FullyQualifiedName~ParityRoadmapDocs_RecordPhase557WhiteboardAnnotationStoreContractGateInBothLocales" --logger "console;verbosity=minimal" (1 passed)
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --no-restore --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests" --logger "console;verbosity=minimal" (57 passed)
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --no-restore --logger "console;verbosity=minimal" (302 passed)
• git diff --check (exit 0; CRLF notices only)

Stack

• Base: PR Phase 556: record whiteboard persistence deferred decision #236 / feature/phase-556-whiteboard-persistence-decision
• Blocked until Phase 556 and the lower whiteboard stack land.
• Lowest known stack blocker remains PR Record Phase 538 eraser feasibility boundary #200 review requirement.

[ExplodingUFO]

Restacked this PR onto the latest Phase 556 head after #236 was rewritten.

Evidence:

• Base: feature/phase-556-whiteboard-persistence-decision at 32921a0eba26046062c2d69cd0be4d8097b97a0f
• Head: 52c02fb8bf03cb8f1a45b547c523fe49eb0f5235
• git range-diff 1dc3aab..5c0469e 32921a0..HEAD: the four Phase 557 commits are equivalent after restack
• Local verification:
  • dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests" -> 57/57 passed
  • dotnet restore AsterGraph.sln -> restored the missing Editor/TestPlugins assets for this worktree
  • dotnet build AsterGraph.sln -c Release --no-restore -> 0 warnings / 0 errors
  • dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --no-build -> 302/302 passed
  • git diff --check 32921a0..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.