fix(audio): use WebRTC VAD on iOS

crates/chanora_audio/src/audio_processing.rs

@@ -100,6 +100,16 @@ pub enum VadBackend {
     Disabled,
 }
 
+#[cfg(target_os = "ios")]
+fn default_vad_backend() -> VadBackend {
+    VadBackend::WebrtcVad
+}
+
+#[cfg(not(target_os = "ios"))]
+fn default_vad_backend() -> VadBackend {
+    VadBackend::SileroOnnx
+}
+
 impl VadBackend {
     /// Stable bridge/debug string.
     pub fn as_str(self) -> &'static str {
@@ -164,7 +174,7 @@ impl Default for AudioProcessingConfig {
             route: AudioRoute::Speaker,
             ios_mode: IosVoiceProcessingMode::PlatformVoiceProcessing,
             processing_backend: AudioBackend::PlatformVoiceProcessing,
-            vad_backend: VadBackend::SileroOnnx,
+            vad_backend: default_vad_backend(),
             aec: EffectOwner::Platform,
             // iOS VPIO owns NS/AGC on the default shipping path. Software
             // effects are opt-in through the experimental raw route only.
@@ -242,6 +252,9 @@ mod tests {
         assert_eq!(config.aec, EffectOwner::Platform);
         assert_eq!(config.ns, EffectOwner::Platform);
         assert_eq!(config.agc, EffectOwner::Platform);
+        #[cfg(target_os = "ios")]
+        assert_eq!(config.vad_backend, VadBackend::WebrtcVad);
+        #[cfg(not(target_os = "ios"))]
         assert_eq!(config.vad_backend, VadBackend::SileroOnnx);
     }