Phase 542: whiteboard primitive model contract gate

[ExplodingUFO]

Summary

• records Phase 542 as the whiteboard primitive core model contract gate in both parity roadmap locales
• adds WHITEBOARD_PRIMITIVE_CORE_MODEL_CONTRACT_GATE and the graph-scene-vs-whiteboard-annotation boundary
• guards the contract with a focused bilingual roadmap-doc test

Stack

• Base: PR Phase 541: post-Phase-540 whiteboard wave split #210 / docs/phase-541-post-whiteboard-wave-split
• Do not merge before PR Phase 541: post-Phase-540 whiteboard wave split #210, PR Phase 540: record whiteboard persistence readiness #204, PR Phase 539: record drawing primitive model gate #202, PR Record Phase 538 eraser feasibility boundary #200, and PR Add hosted lasso pointer mode actions #198 land in order.

Boundaries

• no production model type
• no public API changes
• no runtime UI behavior
• no renderer work
• no persistence/schema migration
• no screenshot manifest expansion
• no toolbar/tool activation
• no eraser behavior
• no full React Flow whiteboard parity claim

Test Plan

• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests.ParityRoadmapDocs_RecordPhase542WhiteboardPrimitiveCoreModelContractGateInBothLocales" --verbosity minimal
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "FullyQualifiedName~ReactFlowParityRoadmapDocsTests" --verbosity minimal
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --verbosity minimal
• git diff --check

Closes #206