fix(audio): inline VAD worker policy for iOS capture callback

PR #37 inlined VadWorkerPolicy into the desktop capture path but missed
the iOS files. Instead of restoring the deleted types, inline the same
direct if-let-Some pattern into ios_voice_unit.rs (the only remaining
iOS backend) and permanently remove VadWorkerPolicy and
callback_vad_worker_policy from vad/mod.rs.

iOS uses Option<AppleCoreMlVadWorker> with &mut self (no Mutex), so the
inline is simpler than the desktop's try_lock pattern.

crates/chanora_audio/src/ios_voice_unit.rs

@@ -406,16 +406,7 @@ impl IosCaptureState {
                     speech: true,
                 }
             } else if vad_backend == crate::VadBackend::SileroOnnx {
-                match crate::vad::callback_vad_worker_policy(
-                    voice_activity_mode,
-                    vad_backend,
-                    self.silero_coreml_worker.is_some(),
-                ) {
-                    crate::vad::VadWorkerPolicy::UseWorker => {
-                        let worker = self
-                            .silero_coreml_worker
-                            .as_ref()
-                            .expect("policy checked worker");
+                if let Some(worker) = self.silero_coreml_worker.as_ref() {
                     let enqueued = worker.try_send(capture_seq, &frame);
                     if !worker.is_stale(capture_seq) {
                         let p = worker.latest_probability();
@@ -436,8 +427,7 @@ impl IosCaptureState {
                             &frame,
                         )
                     }
-                    }
-                    crate::vad::VadWorkerPolicy::UseFallback => {
+                } else {
                     used_fallback_vad = true;
                     self.mark_vad_fallback_active(crate::VadBackend::SileroOnnx);
                     crate::vad::VoiceActivityDetector::process_10ms(
@@ -445,11 +435,6 @@ impl IosCaptureState {
                         &frame,
                     )
                 }
-                    crate::vad::VadWorkerPolicy::NotModelBacked => crate::vad::VadOutput {
-                        probability: 1.0,
-                        speech: true,
-                    },
-                }
             } else {
                 crate::vad::VoiceActivityDetector::process_10ms(&mut self.vad_detector, &frame)
             };