fix(audio): address CoreML VAD review feedback

crates/chanora_audio/src/audio_processing.rs

@@ -100,12 +100,6 @@ pub enum VadBackend {
     Disabled,
 }
 
-#[cfg(target_os = "ios")]
-fn default_vad_backend() -> VadBackend {
-    VadBackend::SileroOnnx
-}
-
-#[cfg(not(target_os = "ios"))]
 fn default_vad_backend() -> VadBackend {
     VadBackend::SileroOnnx
 }
@@ -114,6 +108,9 @@ impl VadBackend {
     /// Stable bridge/debug string.
     pub fn as_str(self) -> &'static str {
         match self {
+            #[cfg(any(target_os = "ios", target_os = "macos"))]
+            Self::SileroOnnx => "apple_coreml",
+            #[cfg(not(any(target_os = "ios", target_os = "macos")))]
             Self::SileroOnnx => "silero_vad_onnx",
             Self::WebrtcVad => "webrtc_vad",
             Self::EnergyDebug => "energy_debug",