address CodeRabbit review: export fallback constant, fix test hygiene

- Export ANTIGRAVITY_VERSION_FALLBACK so tests import it instead of
  repeating the literal (single update surface on future bumps)
- Add afterEach(vi.unstubAllGlobals) to prevent stub leaks between tests
- Fix semver comparison to use explicit major/minor checks instead of
  major*100+minor encoding that breaks for minor >= 100