add_uniffi_interface_for_livekit_wakeword.md

livekit-uniffi	minor

Add UniFFI interface for livekit-wakeword

Summary

• Expose WakeWordDetector as a UniFFI Object wrapping WakeWordModel with Mutex for interior mutability
• Export new, load_model, and predict methods across FFI
• Use #[uniffi::remote(Error)] with #[uniffi(flat_error)] for WakeWordError, matching the existing AccessTokenError pattern