fix(audio): create CoreML/ONNX VAD worker on backend switch

Root cause: when VadBackend::SileroOnnx was selected, the capture
callback set the worker to None but never called try_new() to create
a replacement. The desktop path (engine.rs) correctly calls
SileroOnnxVadWorker::try_new(), but the iOS (CoreML) and Android
(ONNX) mobile paths only cleared the old worker without spawning a
new one. The VAD stayed stuck at 'warming up' forever, using the
WebRTC fallback for all voice activity detection.

Fix:
- iOS: call AppleCoreMlVadWorker::try_new() when SileroOnnx backend
  is selected; mark fallback only on failure
- Android: call SileroOnnxVadWorker::try_new() with the model bundle
  path when SileroOnnx backend is selected; same fallback logic

crates/chanora_audio/src/android_voice_unit.rs

@@ -354,10 +354,17 @@ impl AndroidCaptureState {
             self.fallback_warned_backend = None;
             match vad_backend {
                 crate::VadBackend::SileroOnnx => {
-                    self.silero_vad_worker = None;
+                    let model_path = crate::vad::silero_model_bundle_path();
+                    self.silero_vad_worker =
+                        crate::vad::silero_onnx::SileroOnnxVadWorker::try_new(&model_path);
+                    if self.silero_vad_worker.is_some() {
+                        self.fallback_warned_backend = None;
+                        self.audio_processing_stats.set_vad_fallback_active(false);
+                    } else {
                         self.mark_vad_fallback_active(crate::VadBackend::SileroOnnx);
                         self.audio_processing_stats.set_vad_fallback_active(true);
                     }
+                }
                 _ => {
                     self.silero_vad_worker = None;
                 }

crates/chanora_audio/src/ios_voice_unit.rs

@@ -351,9 +351,15 @@ impl IosCaptureState {
             self.current_vad_backend = vad_backend;
             self.fallback_warned_backend = None;
             if vad_backend == crate::VadBackend::SileroOnnx {
-                self.silero_coreml_worker = None;
+                self.silero_coreml_worker =
+                    crate::vad::apple_coreml::AppleCoreMlVadWorker::try_new();
+                if self.silero_coreml_worker.is_some() {
+                    self.fallback_warned_backend = None;
+                    self.audio_processing_stats.set_vad_fallback_active(false);
+                } else {
                     self.mark_vad_fallback_active(crate::VadBackend::SileroOnnx);
                     self.audio_processing_stats.set_vad_fallback_active(true);
+                }
             } else {
                 self.silero_coreml_worker = None;
                 self.audio_processing_stats.set_vad_fallback_active(false);