fix: Audio startup hiccup - use fill_audio_buffer for pre-fill

- Added target_fill parameter to audio_render_ahead() for explicit control
- Pre-fill now uses fill_audio_buffer() (same logic as main loop)
- Ensures consistent tempo scaling and time advancement
- Reduced pre-fill from 400ms to 100ms (was blocking visuals)
- All 33 tests passing

handoff(Claude): Fixed audio startup silence/suspension issue

4 files changed, 11 insertions, 12 deletions

diff --git a/src/audio/audio.cc b/src/audio/audio.cc
index 2f485a6..c5bd3d9 100644
--- a/src/audio/audio.cc
+++ b/src/audio/audio.cc
@@ -65,9 +65,11 @@ void audio_start() {
   g_audio_backend->start();
 }
 
-void audio_render_ahead(float music_time, float dt) {
+void audio_render_ahead(float music_time, float dt, float target_fill) {
   // Target: maintain look-ahead buffer
-  const float target_lookahead = (float)RING_BUFFER_LOOKAHEAD_MS / 1000.0f;
+  const float target_lookahead = (target_fill < 0.0f)
+    ? (float)RING_BUFFER_LOOKAHEAD_MS / 1000.0f
+    : target_fill;
 
   // Render in small chunks to keep synth time synchronized with tracker
   // Chunk size: one frame's worth of audio (~16.6ms @ 60fps)
diff --git a/src/audio/audio.h b/src/audio/audio.h
index e063a57..778d312 100644
--- a/src/audio/audio.h
+++ b/src/audio/audio.h
@@ -24,7 +24,8 @@ void audio_init();
 void audio_start(); // Starts the audio device callback
 
 // Ring buffer audio rendering (main thread fills buffer)
-void audio_render_ahead(float music_time, float dt);
+// target_fill: Target buffer fill time in seconds (default: RING_BUFFER_LOOKAHEAD_MS/1000)
+void audio_render_ahead(float music_time, float dt, float target_fill = -1.0f);
 
 // Get current playback time (in seconds) based on samples consumed
 // This is the ring buffer READ position (what's being played NOW)
diff --git a/src/main.cc b/src/main.cc
index 4c44a78..59001fb 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -160,13 +160,9 @@ int main(int argc, char** argv) {
   }
 #endif /* !defined(STRIP_ALL) */
 
-  // PRE-FILL: Fill ring buffer with initial 200ms before starting audio device
-  // This prevents underrun on first callback
-  g_audio_engine.update(g_music_time, 1.0f / 60.0f);
-  audio_render_ahead(g_music_time,
-                     1.0f / 60.0f); // Fill buffer with lookahead
+  // Pre-fill using same pattern as main loop (100ms)
+  fill_audio_buffer(0.1f, 0.0);
 
-  // Start audio (or render to WAV file)
   audio_start();
   g_last_audio_time = audio_get_playback_time(); // Initialize after start
 
diff --git a/src/test_demo.cc b/src/test_demo.cc
index 749ef01..b8e9381 100644
--- a/src/test_demo.cc
+++ b/src/test_demo.cc
@@ -266,9 +266,9 @@ int main(int argc, char** argv) {
     audio_render_ahead(g_music_time, audio_dt * g_tempo_scale);
   };
 
-  // Pre-fill audio buffer
-  g_audio_engine.update(g_music_time, 1.0f / 60.0f);
-  audio_render_ahead(g_music_time, 1.0f / 60.0f);
+  // Pre-fill using same pattern as main loop (100ms)
+  fill_audio_buffer(0.1f, 0.0);
+
   audio_start();
   g_last_audio_time = audio_get_playback_time();