Phase 562: annotation clipboard/export boundary

[ExplodingUFO]

Summary

• Record Phase 562 as a docs/tests-only whiteboard annotation clipboard/export serialization boundary gate.
• Define annotation identity/reference and primitive payload eligibility across clipboard fragments, scene export artifacts, and workspace sidecar persistence.
• Keep production serialization, clipboard/export behavior changes, file/database I/O, GraphDocument schema/version changes, workspace persistence changes, public API exposure, saved annotation state, and full whiteboard parity explicitly out of scope.

Test Plan

• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj --filter "FullyQualifiedName~ParityRoadmapDocs_RecordPhase562ClipboardExportSerializationBoundaryInBothLocales" (RED before docs update, then GREEN)
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests"
• dotnet build AsterGraph.sln -c Release
• dotnet test AsterGraph.sln
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj
• git diff --check

Stack

• Base: docs/phase-561-whiteboard-annotation-sidecar-persistence-policy / PR Phase 561: annotation workspace sidecar policy #249
• Issue: closes Phase 562: whiteboard annotation clipboard/export serialization boundary #244
• Bead: avalonia-node-map-0gi

[ExplodingUFO]

Phase 562 final stacked handoff after restack:

• Restacked base: 209d0c429fd40f5e8fd1124e0c51165ce62606dd (PR Phase 561: annotation workspace sidecar policy #249 / Phase 561 head)
• Restacked head: 07248c37579cc21c3574ef396729f49a2872681b
• Range-diff check: git range-diff 5e15cd9..backup/phase-562-before-restack-20260514 209d0c4..HEAD shows the single Phase 562 commit as equivalent after replay.
• Local verification:
  • ReactFlowParityRoadmapDocsTests: 62/62 passed
  • Release build: 0 warnings / 0 errors
  • AsterGraph.Demo.Tests: 307/307 passed
  • git diff --check: clean
• Remote verification: PR is MERGEABLE / CLEAN at this head.
• CI: all duplicate status checks passed, including hygiene-proof, framework-matrix net8.0/net9.0/net10.0, linux-validation, macos-validation, contract-proof, and both release-validation jobs.

Still intentionally blocked only by stack order behind PR #249 and lower stack PRs.