feat: add URL type detection + make index.js importable for tests
- guard main() behind entry-point check to allow importing for tests
- add isProfileUrl() to detect profile URLs (rejecting post URLs)
- add resolveProfileTarget() to parse profile URLs and resolve handles to DIDs
- create test.js with comprehensive test coverage for isProfileUrl()
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>