Add whiteboard primitive screenshot proof gate

[ExplodingUFO]

Summary

• Adds the Phase 550 proof-only Cookbook recipe for whiteboard-primitive-screenshot-proof-route.
• Adds scene and full-window shell screenshot manifest rows for cookbook-whiteboard-primitive-screenshot-proof and shell-cookbook-whiteboard-primitive-screenshot-proof.
• Updates bilingual Cookbook and React Flow parity roadmap docs to replace the Phase 550 TBD row with GitHub Phase 550: whiteboard primitive Cookbook screenshot implementation gate #223 / avalonia-node-map-wjr and preserve no-runtime/no-schema/no-drawing-tool boundaries.

Stack / Merge Order

• Stacked on feature/phase-549-whiteboard-persistence-decision / PR Phase 549: whiteboard persistence decision gate #222.
• BLOCKED: do not merge before Phase 549 / PR Phase 549: whiteboard persistence decision gate #222 and the earlier stack land.

Verification

• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests.ParityRoadmapDocs_RecordPhase550WhiteboardPrimitiveCookbookScreenshotGateInBothLocales" -v:minimal
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~DemoCookbookScreenshotGateTests.CookbookScreenshotGate_IncludesPhase550WhiteboardPrimitiveScreenshotProofRoute" -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 --filter "FullyQualifiedName~DemoCookbookDocsTests|FullyQualifiedName~DemoCookbookCatalogTests" -v:minimal
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~DemoCookbookScreenshotGateTests" -v:minimal
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~ShellVisualCoveragePlanningDocsTests" -v:minimal
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~V075LibraryProofClosureTests.PublicDocs_DoNotNameExternalInspirationProjects" -v:minimal
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release -v:minimal (290 passed)
• dotnet build AsterGraph.sln -c Release -v:minimal (0 warnings, 0 errors)
• git diff --check

Notes

• One initial concurrent focused test run hit the known Windows build file-lock path; serial rerun passed.
• The screenshot gate now inspects freshly read PNG bytes instead of reopening the same image by path for pixel inspection, avoiding a fragile headless capture decode path while preserving the same PNG validity and nonblank checks.

Closes #223