Phase 552: add whiteboard drawing tool activation contract

[ExplodingUFO]

Summary

• Add the hosted Avalonia whiteboard drawing activation contract via NodeCanvas.WhiteboardDrawingMode and NodeCanvasWhiteboardDrawingMode.
• Add CreateWhiteboardDrawingToolActions(...) host-source actions for rectangle and freehand activation without runtime command ids.
• Update public API baseline, inventory/recipe/parity docs, and regression tests for the activation-only boundary.

Scope

• Activation only: the actions only set canvas state.
• No pointer gesture capture, runtime drawing creation, toolbar UI implementation, eraser behavior, persistence/schema changes, renderer rewrite, screenshot manifest expansion, or full React Flow whiteboard parity claim.

Verification

• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --filter "HostedActions_WhiteboardDrawingToolActionsSwitchNodeCanvasDrawingModeWithoutRuntimeCommandRoute|CommandRegistry_DoesNotExposeWhiteboardDrawingToolActivationAsRuntimeSessionCommand" --verbosity minimal -> 2 passed
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "ParityRoadmapDocs_RecordPhase552WhiteboardDrawingToolActivationContractInBothLocales|ParityRoadmapDocs_RecordWhiteboardFeasibilityScopeInBothLocales" --verbosity minimal -> 1 passed
• & .\eng\validate-public-api-surface.ps1 -Configuration Release -Framework net9.0 -> PUBLIC_API_SURFACE_OK:4291:net9.0
• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --verbosity minimal -> 868 passed
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --verbosity minimal -> 295 passed
• git diff --cached --check -> clean before commit

Stacked after PR #231 / Phase 551; do not merge before Phase 551.

Closes #226