Add Flare as a third inference engine backend #293

Summary

Add Flare LLM as a third inference engine option alongside MLC WebLLM and Transformers.js. Flare is a pure Rust → WASM inference engine with WebGPU acceleration that loads standard GGUF files directly (no TVM compilation step).

Why

Key advantage: users can grab any GGUF model from HuggingFace and use it immediately — no conversion pipeline.

Integration plan

1. Publish @aspect/flare npm package via wasm-pack
2. Create FlareEngine adapter implementing the BrowserAI engine interface
3. Map BrowserAI API to Flare WASM API:
   • loadModel() → FlareEngine.load() + init_gpu()
   • generateText() → begin_stream() + next_token() loop
   • transcribeAudio() → N/A (Flare doesn't do STT yet)
4. Add GGUF models to model registry
5. Allow engine selection: new BrowserAI({ engine: 'flare' })

Depends on

• #[flare-npm] Publish @aspect/flare npm package
• #[flare-adapter] FlareEngine adapter implementation
• #[flare-models] Add GGUF models to registry