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

1 files changed, 11 insertions, 12 deletions

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