Phase 560: in-memory annotation store adapter proof

[ExplodingUFO]

Summary

• Add internal InMemoryGraphWhiteboardAnnotationStoreBoundary over the Phase 558 annotation-store boundary.
• Cover empty, create/read, update-by-replacement, delete-by-replacement, defensive-read, and session-scoped instance behavior.
• Record Phase 560 evidence in the React Flow parity roadmap without claiming persistence, GraphDocument schema, public API, saved whiteboard state, or full parity.

Test Plan

• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj --filter "FullyQualifiedName~WhiteboardAnnotationStoreContractTests"
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj --filter "FullyQualifiedName~ParityRoadmapDocs_RecordPhase560InMemoryAnnotationStoreAdapterProofInBothLocales"
• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj
• dotnet build AsterGraph.sln -c Release
• dotnet test AsterGraph.sln
• git diff --check (exit 0; LF/CRLF warnings only)

Stack

• Depends on Phase 559: refresh annotation store implementation queue #247 / Phase 559.
• Must not merge before Phase 559 lands.

Closes #242

[ExplodingUFO]

Final restack verification for PR #248 after PR #247 was rewritten:

• Base: d13318e6dea0fe230c5609870938c4862a00c3a4 (docs/phase-559-annotation-store-implementation-queue)
• Head: b1fe5e68f98142a2f34c713978e28ac40e189a8d (feature/phase-560-whiteboard-annotation-store-memory-adapter)
• git range-diff 4b0c228..backup/phase-560-before-restack-20260514 d13318e..HEAD confirms the single Phase 560 implementation commit is equivalent after restack.

Local verification in .worktrees/phase-560-whiteboard-annotation-store-memory-adapter:

• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --filter "FullyQualifiedName~WhiteboardAnnotationStoreContractTests" -> 6/6 passed
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests" -> 60/60 passed after rerun; first concurrent attempt hit a compile file lock, not a test failure
• 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 -> 888/888 passed
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --no-build -> 305/305 passed
• pwsh -NoProfile -ExecutionPolicy Bypass -File eng\validate-public-api-surface.ps1 -Configuration Release -Framework net9.0 -> PUBLIC_API_SURFACE_OK:4292:net9.0 and related scope/guidance/diff markers passed
• git diff --check d13318e6dea0fe230c5609870938c4862a00c3a4..HEAD -> clean

GitHub remote verification is now MERGEABLE / CLEAN; all duplicate checks passed: hygiene-proof, framework-matrix net8.0/net9.0/net10.0, linux-validation, macos-validation, contract-proof, and release-validation.

PR remains blocked only for stacked merge ordering behind Phase 559 / PR #247 and the lower stack.