fix(audio): WAV dump backend now properly triggers tracker patterns

Fixed critical bug where WavDumpBackend rendered only silence (zeros).

Root Cause Analysis:
- WavDumpBackend::start() called synth_render() in a loop
- BUT never called tracker_update() to trigger patterns
- Result: No voices triggered, synth rendered silence (zero-filled WAV)

The Fix:
- Added #include "tracker.h" to wav_dump_backend.cc
- Implemented music time simulation in WavDumpBackend::start()
- Now calls tracker_update(music_time) before each synth_render()
- Simulates tempo scaling phases (matches main.cc logic):
  * 0-10s: tempo = 1.0x (steady)
  * 10-15s: tempo = 1.0 → 2.0x (acceleration)
  * 15-20s: tempo = 1.0x (reset)
  * 20-25s: tempo = 1.0 → 0.5x (deceleration)
  * 25s+: tempo = 1.0x (reset)

Technical Details:
- Calculate dt = kBufferSize / kSampleRate (time per audio buffer)
- Track music_time, physical_time, and tempo_scale
- Advance music_time by dt * tempo_scale each iteration
- Call tracker_update(music_time) to trigger patterns
- Then call synth_render() to render triggered voices

Enhanced Progress Output:
- Now shows: "Rendering: X.Xs / 60s (music: Y.Ys, tempo: Z.ZZx)"
- Final summary includes total music time
- Example: "60.00 seconds, 61.24 music time" (tempo scaling verified)

Verification:
✓ WAV file now contains actual audio data (not zeros)
✓ Hexdump shows varying sample values (37 00, df ff, etc.)
✓ 141,307 non-zero data lines in 3.7 MB file
✓ Tempo scaling visible in progress output
✓ All 16 tests passing (100%)

Before: Zero-filled WAV, no audio
After: Proper drum track with tempo scaling effects

handoff(Claude): WAV dump bug fixed, audio rendering confirmed

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 33 insertions, 3 deletions

diff --git a/src/audio/wav_dump_backend.cc b/src/audio/wav_dump_backend.cc
index 7418a40..fbbbeaa 100644
--- a/src/audio/wav_dump_backend.cc
+++ b/src/audio/wav_dump_backend.cc
@@ -6,6 +6,7 @@
 #if !defined(STRIP_ALL)
 
 #include "synth.h"
+#include "tracker.h"
 #include <assert.h>
 #include <stdio.h>
 #include <string.h>
@@ -49,7 +50,35 @@ void WavDumpBackend::start() {
   const size_t total_samples = kSampleRate * max_duration_sec;
   const size_t total_frames = total_samples / kBufferSize;
 
+  // Music time tracking (matches main.cc logic)
+  float music_time = 0.0f;
+  float tempo_scale = 1.0f;
+  float physical_time = 0.0f;
+  const float dt = (float)kBufferSize / kSampleRate;  // Time per buffer
+
   for (size_t frame = 0; frame < total_frames; ++frame) {
+    // Update tempo scaling (matches main.cc phases)
+    if (physical_time < 10.0f) {
+      tempo_scale = 1.0f;
+    } else if (physical_time < 15.0f) {
+      const float progress = (physical_time - 10.0f) / 5.0f;
+      tempo_scale = 1.0f + progress * 1.0f;  // 1.0 → 2.0
+    } else if (physical_time < 20.0f) {
+      tempo_scale = 1.0f;
+    } else if (physical_time < 25.0f) {
+      const float progress = (physical_time - 20.0f) / 5.0f;
+      tempo_scale = 1.0f - progress * 0.5f;  // 1.0 → 0.5
+    } else {
+      tempo_scale = 1.0f;
+    }
+
+    // Advance music time
+    music_time += dt * tempo_scale;
+    physical_time += dt;
+
+    // Update tracker to trigger patterns
+    tracker_update(music_time);
+
     // Render audio from synth
     synth_render(sample_buffer_.data(), kBufferSize);
 
@@ -69,7 +98,8 @@ void WavDumpBackend::start() {
     // Progress indicator
     if (frame % 100 == 0) {
       const float progress_sec = (float)samples_written_ / kSampleRate;
-      printf("  Rendering: %.1fs / %ds\r", progress_sec, max_duration_sec);
+      printf("  Rendering: %.1fs / %ds (music: %.1fs, tempo: %.2fx)\r",
+             progress_sec, max_duration_sec, music_time, tempo_scale);
       fflush(stdout);
     }
 
@@ -77,8 +107,8 @@ void WavDumpBackend::start() {
     on_frames_rendered(kBufferSize);
   }
 
-  printf("\nWAV dump complete: %zu samples (%.2f seconds)\n", samples_written_,
-         (float)samples_written_ / kSampleRate);
+  printf("\nWAV dump complete: %zu samples (%.2f seconds, %.2f music time)\n",
+         samples_written_, (float)samples_written_ / kSampleRate, music_time);
 
   is_active_ = false;
 }