Phase 555: add whiteboard primitive eraser behavior

[ExplodingUFO]

Summary

• Adds NodeCanvasWhiteboardDrawingMode.Eraser plus the host-source whiteboard-drawing.eraser action projected as PART_WhiteboardEraserButton.
• Routes eraser-mode left press through transient whiteboard primitive hit-testing/removal without invoking graph selection.delete or deleting selected graph nodes.
• Updates Cookbook, public API inventory, parity docs, and focused tests for action activation, toolbar projection, hit/no-hit eraser behavior, and docs evidence.

Verification

• dotnet test tests\AsterGraph.Editor.Tests\AsterGraph.Editor.Tests.csproj -c Release --verbosity minimal (880 passed)
• dotnet test tests\AsterGraph.Demo.Tests\AsterGraph.Demo.Tests.csproj -c Release --verbosity minimal (300 passed)
• ./eng/validate-public-api-surface.ps1 -Configuration Release -Framework net9.0
• git diff --check

Stack / Merge Order

• Closes Phase 555: whiteboard primitive eraser behavior route #229 / bead avalonia-node-map-71c.
• Stacked on PR Phase 554: whiteboard authoring toolbar and Cookbook UX #234 / feature/phase-554-whiteboard-authoring-cookbook.
• Do not merge before PR Phase 554: whiteboard authoring toolbar and Cookbook UX #234 and the lower stack land; PR Record Phase 538 eraser feasibility boundary #200 is still blocked on required human review.

[ExplodingUFO]

Pre-merge review finding: this PR extends the same tool-mode exclusivity bug from PR #234 to Eraser. The new test asserts that after clicking PART_WhiteboardEraserButton and then PART_PointerModeLassoButton, SelectionMode is Lasso but WhiteboardDrawingMode remains Eraser. Because the pointer coordinator handles WhiteboardDrawingMode.Eraser before selection, the canvas still erases/handles whiteboard primitives instead of running lasso selection.

Please make pointer selection modes and whiteboard modes mutually exclusive, or introduce one canonical active tool state. The fix should cover Rectangle, Freehand, and Eraser, with regression tests proving switching back to Marquee/Lasso clears the whiteboard mode and restores normal selection gestures.

[ExplodingUFO]

Fixed the eraser-mode exclusivity blocker and restacked this PR onto the latest Phase 554 head.

What changed:

• PR now targets Phase 554 head ac6b244 instead of the old 6d3cac2 base.
• Kept the Phase 554 shared tool-mode fix for pointer/whiteboard mutual exclusivity.
• Added Eraser-specific regression coverage proving pointer selection clears stale WhiteboardDrawingMode.Eraser.

Verification:

• Red tests reproduced the blocker before the fix: pointer selection left Eraser active and eraser activation did not restore Marquee from Lasso.
• Post-restack focused action/view tests: 51/51 passing.
• dotnet build AsterGraph.sln -c Release -v:minimal: 0 warnings, 0 errors.
• Full AsterGraph.Editor.Tests: 880/880 passing.
• Full AsterGraph.Demo.Tests: 300/300 passing.
• git diff --check: clean.
• GitHub PR checks are all green; PR reports MERGEABLE/CLEAN.