Stabilize loopback prompt timing

app.js

@@ -113,7 +113,8 @@ const WEBGPU_ONLY_REASONS = new Map([
 const DEFAULT_VAD_SILENCE_MS = 480;
 const MIN_VAD_SILENCE_MS = 200;
 const MAX_VAD_SILENCE_MS = 800;
-const DEFAULT_LOOPBACK_SPEED = 1.10;
+const DEFAULT_LOOPBACK_SPEED = 1.0;
+const DEFAULT_LOOPBACK_PREROLL_MS = 180;
 const MIC_START_TIMEOUT_MS = 15000;
 const WEBGPU_ADAPTER_TIMEOUT_MS = 5000;
 const DEFAULT_TTS_CHUNKING = Object.freeze({
@@ -2276,6 +2277,13 @@ function feedLoopbackAudio(audio, sampleRate) {
     }, delayMs);
     delayMs += (chunk.length / sampleRate) * 1000;
   };
+  const prerollSamples = Math.ceil((sampleRate * DEFAULT_LOOPBACK_PREROLL_MS) / 1000);
+  if (prerollSamples > 0) {
+    if (state.activeBenchmark) state.activeBenchmark.loopbackPrerollMs = DEFAULT_LOOPBACK_PREROLL_MS;
+    for (let offset = 0; offset < prerollSamples; offset += chunkSize) {
+      postChunk(new Float32Array(Math.min(chunkSize, prerollSamples - offset)));
+    }
+  }
   for (let offset = 0; offset < samples.length; offset += chunkSize) {
     const chunk = samples.slice(offset, Math.min(offset + chunkSize, samples.length));
     postChunk(chunk);