Feat: background head-preload for sample libraries

src/engine/audio.rs

@@ -251,7 +251,15 @@ pub fn build_stream(
     metrics: Arc<EngineMetrics>,
     initial_samples: Vec<doux::sampling::SampleEntry>,
     audio_sample_pos: Arc<AtomicU64>,
-) -> Result<(Stream, AudioStreamInfo, AnalysisHandle), String> {
+) -> Result<
+    (
+        Stream,
+        AudioStreamInfo,
+        AnalysisHandle,
+        Arc<doux::SampleRegistry>,
+    ),
+    String,
+> {
     let device = match &config.output_device {
         Some(name) => doux::audio::find_output_device(name)
             .ok_or_else(|| format!("Device not found: {name}"))?,
@@ -287,6 +295,7 @@ pub fn build_stream(
     let mut engine =
         Engine::new_with_metrics(sample_rate, channels, max_voices, Arc::clone(&metrics));
     engine.sample_index = initial_samples;
+    let registry = Arc::clone(&engine.sample_registry);
 
     let (mut fft_producer, analysis_handle) = spawn_analysis_thread(sample_rate, spectrum_buffer);
 
@@ -356,5 +365,5 @@ pub fn build_stream(
         host_name,
         channels: effective_channels,
     };
-    Ok((stream, info, analysis_handle))
+    Ok((stream, info, analysis_handle, registry))
 }