Phase 554: whiteboard authoring toolbar and Cookbook UX

[ExplodingUFO]

Summary

• Project rectangle/freehand whiteboard drawing actions into the hosted GraphEditorView authoring toolbar.
• Add the Phase 554 Cookbook route plus screenshot/shell visual manifests for whiteboard-authoring-cookbook-route.
• Update EN/ZH Cookbook, parity, and public API docs while keeping full whiteboard parity, eraser, schema, renderer, and model redesign out of scope.

Test Plan

• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --verbosity minimal (877/877)
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --verbosity minimal (299/299)
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "Cookbook|ReactFlowParityRoadmapDocsTests" --verbosity minimal (140/140)
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --filter "ShellVisualCoveragePlanningDocsTests|PublicDocs_DoNotNameExternalInspirationProjects|ParityRoadmapDocs_RecordPhase554WhiteboardAuthoringCookbookUxRouteInBothLocales|CookbookCatalog_AddsWhiteboardAuthoringCookbookRoute" --verbosity minimal (10/10)
• .\eng\validate-public-api-surface.ps1 -Configuration Release -Framework net9.0
• git diff --check
• public-doc prohibited-term grep for react flow|xyflow|svelte flow|@xyflow|xy flow in README/public launch/Cookbook docs (no matches)

Stack / Merge Order

• Stacked on PR Phase 553: add whiteboard primitive pointer gesture capture #233 / feature/phase-553-whiteboard-pointer-gestures.
• Do not merge before Phase 553 lands.

Closes #228.

[ExplodingUFO]

Pre-merge review finding: the new whiteboard drawing toolbar state is not mutually exclusive with the existing pointer selection mode. The test currently proves the problem: after clicking Freehand and then PART_PointerModeLassoButton, SelectionMode becomes Lasso but WhiteboardDrawingMode remains Freehand. In the actual pointer coordinator from PR #233, whiteboard drawing is checked before canvas selection, so the next left-drag will still create a whiteboard primitive instead of performing lasso selection.

This makes the UI misleading: the user appears to have switched back to lasso/selection, but the active whiteboard mode still intercepts the canvas. Please clear WhiteboardDrawingMode when selecting Marquee/Lasso, or model the tool mode as a single mutually exclusive state. Add a regression test that switching from Rectangle/Freehand back to Lasso/Marquee makes subsequent canvas drag follow selection rather than whiteboard drawing.

[ExplodingUFO]

Fixed the mutual-exclusion blocker in ac6b244.

What changed:

• Pointer/selection actions now clear WhiteboardDrawingMode back to None.
• Whiteboard drawing actions now reset SelectionMode back to Marquee.
• Added regression coverage for both stale-mode directions.

Verification:

• Red tests reproduced the blocker first: pointer action left Freehand active, and drawing action left Lasso active.
• Focused action/view regression tests: 7/7 passing.
• dotnet build AsterGraph.sln -c Release -v:minimal: 0 warnings, 0 errors.
• Full AsterGraph.Editor.Tests: 877/877 passing.
• Full AsterGraph.Demo.Tests: 299/299 passing.
• git diff --check: no whitespace errors; CRLF notices only.
• GitHub PR checks are now all green after rerunning the transient duplicate contract-proof artifact-upload failure.