Port the timeout fixes from PR #351 that resolved the unit-test CI
timeouts. Add HEAVY_TEST_TIMEOUT_MS (30s) in tests/helpers/test-utils.ts
and apply it to the slow gold-preview.fig fixture tests/hooks:
- clipboard roundtrip
- group reclassification
- glyph blob preservation
- auto-layout text measurement
- render/canvas/cache
- Move newly split tests under explicit domain subfolders
- Update nested helper imports after the moves
- Add a lint guard against repeating existing sibling domains as filename prefixes
