fix(tools): support contour segmentation undo and redo

[sedghi]

Summary

• preserve contour-segmentation state across undo/redo after boolean stroke processing replaces the source annotation
• add Undo/Redo controls and standard keyboard shortcuts to the planar freehand contour segmentation example
• add HistoryMemo unit coverage and a browser-state regression for contour merge → undo → redo
• update the GenericViewport test harness to use the current RenderBackends enum name

Root cause

The planar freehand tool initially records an annotation-creation memo for the source stroke. Contour boolean processing then removes that source annotation and rebuilds the resulting contour annotations, leaving the memo pointed at an annotation that no longer exists. Undo therefore could not remove the committed contour, and redo could restore the obsolete source stroke.

This change replaces that provisional memo in-place with a memo that swaps the complete pre-stroke and post-stroke annotation sets, preserving a single undo step for creation, merges, and holes.

Validation

• corepack pnpm run test:vitest:browser tests/vitest-browser/undoRedoHistory.browser.test.ts — 9 passed, 1 expected failure
• corepack pnpm exec jest packages/core/test/utilities/historyMemo.jest.js --runInBand — 5 passed
• corepack pnpm exec tsc --project packages/core/tsconfig.json
• corepack pnpm exec tsc --project packages/tools/tsconfig.json
• Prettier, oxlint, and git diff --check

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/fix-contour-segmentation-undo-redo

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