Phase 541: post-Phase-540 whiteboard wave split

[ExplodingUFO]

Summary

• records Phase 541 as the post-Phase-540 whiteboard implementation wave split in both parity roadmap locales
• assigns tracker-backed follow-up gates for Phases 542-545 across model, renderer/hit-testing, persistence/schema, and Cookbook/screenshot proof
• adds a focused roadmap-doc regression test for the Phase 541 split and concrete follow-up rows

Stack

• Base: PR Phase 540: record whiteboard persistence readiness #204 / docs/phase-540-whiteboard-persistence-render-gate
• Do not merge before 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 runtime behavior changes
• no public API changes
• no model/schema changes
• no renderer-layer changes
• no screenshot manifest expansion
• 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_RecordPhase541PostPhase540WhiteboardWaveSplitInBothLocales" --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 #205