Implement first TTS plugin using new Plugin Architecture:
- plugins/kokoro/: Plugin package with manifest and entry point
- plugins/kokoro/engine.py: KokoroEngine and KokoroSession adapters
- Wraps existing KokoroBackend without modifying it
- Implements VoiceLister capability
- Satisfies Engine/EngineSession protocol
- Passes all 163 contract tests
Tests:
- Plugin loading through Plugin Loader
- Manifest validation
- Engine creation and lifecycle
- Session synthesis and dispose
- VoiceLister capability
15 new tests for Kokoro plugin.
