feat(audio): Add --tempo flag for variable tempo testing

- test_demo: Accelerate [2s->4s], decelerate [6s->8s]
- demo64k: Same tempo logic behind --tempo flag
- Enhanced debug output to show tempo scale and music time

2 files changed, 43 insertions, 37 deletions

diff --git a/src/main.cc b/src/main.cc
index 95582e8..b2b62a8 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -30,6 +30,7 @@ int main(int argc, char** argv) {
   int width = 1280;
   int height = 720;
   bool dump_wav = false;
+  bool tempo_test_enabled = false;
   const char* wav_output_file = "audio_dump.wav";
 
 #if !defined(STRIP_ALL)
@@ -54,6 +55,8 @@ int main(int argc, char** argv) {
       if (i + 1 < argc && argv[i + 1][0] != '-') {
         wav_output_file = argv[++i];
       }
+    } else if (strcmp(argv[i], "--tempo") == 0) {
+      tempo_test_enabled = true;
     }
   }
 #else
@@ -92,30 +95,26 @@ int main(int argc, char** argv) {
   static float g_last_audio_time = 0.0f;
 
   auto fill_audio_buffer = [&](float audio_dt, double physical_time) {
-    // Variable tempo system - acceleration phases for demo effect
-    // Phase 1 (0-5s): Steady 1.0x
-    // Phase 2 (5-10s): Steady 1.0x
-    // Phase 3 (10-15s): Accelerate from 1.0x to 2.0x
-    // Phase 4 (15-20s): Steady 1.0x (reset after acceleration)
-    // Phase 5 (20-25s): Decelerate from 1.0x to 0.5x
-    // Phase 6 (25s+): Steady 1.0x (reset after deceleration)
-    const float prev_tempo = g_tempo_scale;
-    if (physical_time < 10.0) {
-      g_tempo_scale = 1.0f; // Steady at start
-    } else if (physical_time < 15.0) {
-      // Phase 3: Linear acceleration
-      const float progress = (float)(physical_time - 10.0) / 5.0f;
-      g_tempo_scale = 1.0f + progress * 1.0f; // 1.0 → 2.0
-    } else if (physical_time < 20.0) {
-      g_tempo_scale = 1.0f; // Reset to normal
-    } else if (physical_time < 25.0) {
-      // Phase 5: Linear deceleration
-      const float progress = (float)(physical_time - 20.0) / 5.0f;
-      g_tempo_scale = 1.0f - progress * 0.5f; // 1.0 → 0.5
+    // Calculate tempo scale if --tempo flag enabled
+    if (tempo_test_enabled) {
+      const float t = (float)physical_time;
+      if (t >= 2.0f && t < 4.0f) {
+        // [2s->4s]: Accelerate from 1.0x to 1.5x
+        const float progress = (t - 2.0f) / 2.0f;
+        g_tempo_scale = 1.0f + (0.5f * progress);
+      } else if (t >= 6.0f && t < 8.0f) {
+        // [6s->8s]: Decelerate from 1.0x to 0.66x
+        const float progress = (t - 6.0f) / 2.0f;
+        g_tempo_scale = 1.0f - (0.34f * progress);
+      } else {
+        // All other times: Normal tempo
+        g_tempo_scale = 1.0f;
+      }
     } else {
-      g_tempo_scale = 1.0f; // Reset to normal
+      g_tempo_scale = 1.0f; // No tempo variation
     }
-    g_tempo_scale = 1.0f;
+
+    const float prev_tempo = g_tempo_scale;
 
 #if !defined(STRIP_ALL)
     // Debug output when tempo changes significantly
@@ -276,9 +275,17 @@ int main(int argc, char** argv) {
     static float last_graphics_print_time = -1.0f;
     if (current_physical_time - last_graphics_print_time >=
         0.5f) { // Print every 0.5 seconds
-      printf("[GraphicsT=%.2f, AudioT=%.2f, Beat=%d, Frac=%.2f, Peak=%.2f]\n",
-             current_physical_time, current_audio_time, beat_number, beat,
-             visual_peak);
+      if (tempo_test_enabled) {
+        printf(
+            "[GraphicsT=%.2f, AudioT=%.2f, MusicT=%.2f, Beat=%d, Frac=%.2f, "
+            "Peak=%.2f, Tempo=%.2fx]\n",
+            current_physical_time, current_audio_time, g_music_time,
+            beat_number, beat, visual_peak, g_tempo_scale);
+      } else {
+        printf("[GraphicsT=%.2f, AudioT=%.2f, Beat=%d, Frac=%.2f, Peak=%.2f]\n",
+               current_physical_time, current_audio_time, beat_number, beat,
+               visual_peak);
+      }
       last_graphics_print_time = current_physical_time;
     }
 
diff --git a/src/test_demo.cc b/src/test_demo.cc
index 656d0ba..91c5427 100644
--- a/src/test_demo.cc
+++ b/src/test_demo.cc
@@ -229,19 +229,18 @@ int main(int argc, char** argv) {
   auto fill_audio_buffer = [&](float audio_dt, double physical_time) {
     // Calculate tempo scale if --tempo flag enabled
     if (tempo_test_enabled) {
-      // Each bar = 2 seconds at 120 BPM (4 beats)
-      const float bar_duration = 2.0f;
-      // Use physical_time for tempo modulation progression
-      const int bar_number = (int)(physical_time / bar_duration);
-      const float bar_progress = fmodf((float)physical_time, bar_duration) /
-                                 bar_duration; // 0.0-1.0 within bar
-
-      if (bar_number % 2 == 0) {
-        // Even bars: Ramp from 1.0x → 1.5x
-        g_tempo_scale = 1.0f + (0.5f * bar_progress);
+      const float t = (float)physical_time;
+      if (t >= 2.0f && t < 4.0f) {
+        // [2s->4s]: Accelerate from 1.0x to 1.5x
+        const float progress = (t - 2.0f) / 2.0f;
+        g_tempo_scale = 1.0f + (0.5f * progress);
+      } else if (t >= 6.0f && t < 8.0f) {
+        // [6s->8s]: Decelerate from 1.0x to 0.66x
+        const float progress = (t - 6.0f) / 2.0f;
+        g_tempo_scale = 1.0f - (0.34f * progress);
       } else {
-        // Odd bars: Ramp from 1.0x → 0.66x
-        g_tempo_scale = 1.0f - (0.34f * bar_progress);
+        // All other times: Normal tempo
+        g_tempo_scale = 1.0f;
       }
     } else {
       g_tempo_scale = 1.0f; // No tempo variation