- Replace clipboard, variable, centerline, layout, menu, and text formatting assertions with explicit guards
- Use expectDefined and getNodeOrThrow instead of postfix non-null assertions
- Validate affected engine and E2E tests
- Move clipboard.test.ts into focused import, bounds, HTML encoding, and fixture roundtrip files
- Add a shared clipboard graph helper for import tests
- Preserve all 29 clipboard tests after the split
