Phase 548: whiteboard primitive renderer adapter skeleton

[ExplodingUFO]

Summary

• Add internal Core renderer-neutral whiteboard primitive scene snapshot, scene item, hit-test result, and adapter projection skeleton.
• Prove rectangle/freehand projection, topmost z-index hit testing, edit-lifecycle evidence, and separation from graph scene/Avalonia renderer surfaces.
• Update EN/ZH React Flow parity roadmap docs to replace Phase 548 TBD with GitHub Phase 548: whiteboard primitive renderer adapter skeleton #219 / avalonia-node-map-10p while leaving Phase 549/550 TBD.

Test Plan

• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --filter WhiteboardPrimitiveRendererAdapterContractsTests -v:minimal
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests" -v:minimal
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release -v:minimal
• dotnet build AsterGraph.sln -c Release -v:minimal
• git diff --check

Stack

• Stacked after PR Phase 547: whiteboard primitive model skeleton #218 / Phase 547.
• Do not merge before Phase 547 and the earlier whiteboard stack land.