Add voices field to TTSBackendMetadata so each backend's supported
voice list is part of its metadata rather than external constants.
- Add voices: tuple[str, ...] = () to TTSBackendMetadata
- Create _KOKORO_METADATA / _SUPERTONIC_METADATA as single source
of truth for both metadata property and registry registration
- Update KokoroBackend.get_available_voices() to use self.metadata.voices
- Update SupertonicBackend.get_available_voices() to use self.metadata.voices
- Add tests for voices field, metadata voice content, and unified instance identity
- Add create_kokoro_backend() factory in kokoro.py
- Add create_supertonic_backend() factory in supertonic.py
- Auto-discover backend modules in __init__.py via pkgutil
- Both backends register themselves on import
- Tests verify registration and factory callables
- TTSBackendRegistry class with register(), list_backends(), get_metadata(), create_backend()
- Global registry singleton with register_backend() and create_backend() convenience functions
- Unit tests for registry operations
Artem Akymenko