fix(audio): harden realtime callback paths

crates/chanora_audio/src/vad/mod.rs

@@ -15,7 +15,7 @@ use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::{OnceLock, RwLock};
 
 use crate::frame::{f32_to_i16, i16_to_f32};
-use crate::AudioError;
+use crate::{AudioError, VadBackend};
 use resampler::{Downsampler48to16, INPUT_FRAME_10MS};
 
 #[cfg(not(target_os = "ios"))]
@@ -108,6 +108,36 @@ pub fn process_i16_10ms(detector: &mut dyn VoiceActivityDetector, samples: &[i16
     detector.process_10ms(&frame)
 }
 
+/// Callback-side policy for optional model-backed VAD workers.
+#[derive(Debug, Clone, Copy, Eq, PartialEq)]
+pub(crate) enum VadWorkerPolicy {
+    /// Keep using the already-available model worker.
+    UseWorker,
+    /// No worker may be constructed on the callback thread; use WebRTC fallback.
+    UseFallback,
+    /// This backend does not need a model worker.
+    NotModelBacked,
+}
+
+/// Decide whether a realtime callback may use a model-backed VAD worker.
+///
+/// Model/worker construction is intentionally absent from this policy: if a
+/// worker is not already present, callbacks must stay nonblocking and fall back.
+pub(crate) fn callback_vad_worker_policy(
+    voice_activity_mode: bool,
+    backend: VadBackend,
+    worker_available: bool,
+) -> VadWorkerPolicy {
+    if !voice_activity_mode || backend != VadBackend::SileroOnnx {
+        return VadWorkerPolicy::NotModelBacked;
+    }
+    if worker_available {
+        VadWorkerPolicy::UseWorker
+    } else {
+        VadWorkerPolicy::UseFallback
+    }
+}
+
 static SILERO_MODEL_PATH_OVERRIDE: OnceLock<RwLock<Option<String>>> = OnceLock::new();
 static SILERO_MODEL_EPOCH: AtomicU64 = AtomicU64::new(0);
 
@@ -252,4 +282,32 @@ mod tests {
         assert_eq!(silero_model_bundle_path(), path.to_string_lossy());
         let _ = std::fs::remove_file(path);
     }
+
+    #[test]
+    fn callback_policy_uses_existing_model_worker_only() {
+        assert_eq!(
+            callback_vad_worker_policy(true, VadBackend::SileroOnnx, true),
+            VadWorkerPolicy::UseWorker
+        );
+        assert_eq!(
+            callback_vad_worker_policy(true, VadBackend::SileroOnnx, false),
+            VadWorkerPolicy::UseFallback
+        );
+    }
+
+    #[test]
+    fn callback_policy_keeps_disabled_and_webrtc_paths_worker_free() {
+        assert_eq!(
+            callback_vad_worker_policy(false, VadBackend::SileroOnnx, false),
+            VadWorkerPolicy::NotModelBacked
+        );
+        assert_eq!(
+            callback_vad_worker_policy(true, VadBackend::Disabled, false),
+            VadWorkerPolicy::NotModelBacked
+        );
+        assert_eq!(
+            callback_vad_worker_policy(true, VadBackend::WebrtcVad, false),
+            VadWorkerPolicy::NotModelBacked
+        );
+    }
 }