Add Gemini Flash TTS engine support

[OwenMcGirr]

Summary

• Adds a new gemini engine for Gemini Flash TTS via direct Gemini REST API calls.
• Supports the Flash TTS preview models, static Gemini voice metadata, WAV output by default, and raw PCM via format: "pcm".
• Registers the engine in Node/browser factories, exports, provider types, CLI install help, test runner scripts, and README docs.

Closes #53

Testing

• npm run test -- gemini
• npm run test
• npm run build
• Manual smoke test in sibling project generated and played gemini-output.wav using Gemini Flash TTS.