Phase 543: whiteboard renderer hit-testing proof gate

[ExplodingUFO]

Summary

• records Phase 543 as the whiteboard renderer projection and hit-testing proof gate in both parity roadmap locales
• adds WHITEBOARD_RENDERER_HIT_TEST_PROOF_GATE and the scene snapshot / renderer seam / hit-testing proof boundary
• guards the contract with a focused bilingual roadmap-doc test

Stack

• Base: PR Phase 542: whiteboard primitive model contract gate #211 / feature/phase-542-whiteboard-primitive-core-model
• Do not merge before PR Phase 542: whiteboard primitive model contract gate #211, 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 renderer rewrite
• no new persistence/schema behavior
• no screenshot manifest expansion
• no public drawing API
• 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_RecordPhase543WhiteboardRendererHitTestingProofGateInBothLocales" --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 #207