Phase 564: whiteboard annotation migration and compatibility proof

[ExplodingUFO]

Summary

• Adds the Phase 564 docs/tests planning gate for WHITEBOARD_ANNOTATION_MIGRATION_COMPATIBILITY_PROOF.
• Defines annotation-store versions, primitive references, style/geometry payloads, migration metadata validation, and compatibility coverage required before persistence, clipboard/export, or screenshot proof expands.
• Keeps the slice docs/tests-only: no production migration implementation, persisted annotation state, file/database I/O, GraphDocument schema/version changes, workspace persistence, clipboard/export behavior, screenshot manifest, renderer/pointer, public API, or full parity claim.

Verification

• RED: dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests.ParityRoadmapDocs_RecordPhase564MigrationCompatibilityProofInBothLocales" failed before docs update on missing marker WHITEBOARD_ANNOTATION_MIGRATION_COMPATIBILITY_PROOF.
• GREEN focused: same command passed 1/1 after docs update.
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests" passed 64/64.
• git diff --check exited 0 with only LF-to-CRLF warnings.
• dotnet build AsterGraph.sln -c Release succeeded with 0 warnings/errors.
• dotnet test AsterGraph.sln passed Editor.Tests 887/887 and Demo.Tests 309/309.
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj passed 309/309.

Stacked after PR #251 / Phase 563. Do not merge before lower stack lands.

[ExplodingUFO]

Phase 564 final stacked handoff after restack:

• Restacked base: 614d07817fa4330d6303753d2bbc778f07082432 (PR Phase 563: whiteboard annotation screenshot and Cookbook proof expansion #251 / Phase 563 head)
• Restacked head: ab5263c38179d95cca7d42f0dea446652a03b289
• Range-diff check: git range-diff b00cdf9..backup/phase-564-before-restack-20260514 614d078..HEAD shows the single Phase 564 commit as equivalent after replay.
• Local verification:
  • ReactFlowParityRoadmapDocsTests: 64/64 passed
  • Release build: 0 warnings / 0 errors
  • AsterGraph.Demo.Tests: 309/309 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 #251 and lower stack PRs.