Add changeset for livekit-wakeword UniFFI interface

Author: pham-tuan-binh

.changeset/add_uniffi_interface_for_livekit_wakeword.md

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+---
+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