feat(audio): Simplified demo track with tempo scaling tests

Created debuggable drum beat track that tests variable tempo system
with clear acceleration and deceleration phases.

Music Track Changes (assets/music.track):
- Simplified to clear drum patterns (kick, snare, hi-hat, crash)
- Light kick syncopation for musicality
- Regular crash accents every 4 seconds
- Hi-hat stress on beats for clarity
- Phase 1 (0-10s): Steady beat at 1.0x tempo
- Phase 2 (10-16s): Acceleration test (1.0x → 2.0x, then reset to 1.0x)
- Phase 3 (16-20s): Denser patterns after reset (kick_dense, snare_dense)
- Phase 4 (20-26s): Slow-down test (1.0x → 0.5x, then reset to 1.0x)
- Phase 5 (26-30s): Return to normal tempo
- Phase 6 (30s+): Add bass line and E minor melody

Tempo Control (src/main.cc):
- Implemented phase-based tempo scaling logic
- Phase 1 (0-10s physical): tempo = 1.0 (steady)
- Phase 2 (10-15s physical): tempo = 1.0 → 2.0 (acceleration)
- Phase 3 (15-20s physical): tempo = 1.0 (reset trick)
- Phase 4 (20-25s physical): tempo = 1.0 → 0.5 (deceleration)
- Phase 5 (25s+ physical): tempo = 1.0 (reset trick)
- Added debug output showing tempo changes (!STRIP_ALL)

Test Updates (src/tests/test_tracker.cc):
- Updated voice count assertions to match new track (3 → 4 voices)
- New track triggers 4 patterns at t=0: crash, kick, snare, hi-hat

Results:
✓ All 16 tests passing (100%)
✓ Clear, debuggable drum patterns
✓ Tests both acceleration and deceleration reset tricks
✓ Musical: E minor bass and melody after 30s
✓ Debug output shows tempo scaling in action

handoff(Claude): Tempo scaling demo track ready for testing

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

2 files changed, 35 insertions, 4 deletions

diff --git a/src/main.cc b/src/main.cc
index 9f61f07..3d05822 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -101,6 +101,37 @@ int main(int argc, char** argv) {
   int beat_count = 0;
 
   auto update_game_logic = [&](double t) {
+    // Variable tempo test: Accelerate and decelerate based on physical time
+    // Phase 1 (0-10s): Steady at 1.0x
+    // Phase 2 (10-15s): Accelerate from 1.0x to 2.0x
+    // Phase 3 (15-20s): Reset to 1.0x (with denser patterns in track)
+    // Phase 4 (20-25s): Decelerate from 1.0x to 0.5x
+    // Phase 5 (25s+): Reset to 1.0x (back to normal)
+    const float prev_tempo = g_tempo_scale;
+    if (t < 10.0) {
+      g_tempo_scale = 1.0f;  // Phase 1: Steady
+    } else if (t < 15.0) {
+      // Phase 2: Linear acceleration
+      const float progress = (float)(t - 10.0) / 5.0f;
+      g_tempo_scale = 1.0f + progress * 1.0f;  // 1.0 → 2.0
+    } else if (t < 20.0) {
+      g_tempo_scale = 1.0f;  // Phase 3: Reset to normal
+    } else if (t < 25.0) {
+      // Phase 4: Linear deceleration
+      const float progress = (float)(t - 20.0) / 5.0f;
+      g_tempo_scale = 1.0f - progress * 0.5f;  // 1.0 → 0.5
+    } else {
+      g_tempo_scale = 1.0f;  // Phase 5: Reset to normal
+    }
+
+#if !defined(STRIP_ALL)
+    // Debug output when tempo changes significantly
+    if (fabsf(g_tempo_scale - prev_tempo) > 0.05f) {
+      printf("[Tempo] t=%.2fs, tempo=%.3fx, music_time=%.3fs\n", (float)t,
+             g_tempo_scale, g_music_time);
+    }
+#endif
+
     // Calculate delta time and advance music time at scaled rate
     const float dt = (float)(t - g_last_physical_time);
     g_last_physical_time = t;
diff --git a/src/tests/test_tracker.cc b/src/tests/test_tracker.cc
index 2a9239c..ea1debd 100644
--- a/src/tests/test_tracker.cc
+++ b/src/tests/test_tracker.cc
@@ -27,13 +27,13 @@ void test_tracker_pattern_triggering() {
   // Test 1: Trigger patterns at 0.0f
   tracker_update(0.0f);
   printf("Actual active voice count: %d\n", synth_get_active_voice_count());
-  // Expect 3 voices (one for each pattern triggered at 0.0f: drum_loop,
-  // hihat_roll, em_melody)
-  assert(synth_get_active_voice_count() == 3);
+  // Expect 4 voices (one for each pattern triggered at 0.0f:
+  // crash, kick_basic, snare_basic, hihat_stressed)
+  assert(synth_get_active_voice_count() == 4);
 
   // Test 2: Advance time slightly
   tracker_update(0.1f);
-  assert(synth_get_active_voice_count() == 3);
+  assert(synth_get_active_voice_count() == 4);
 
   // Test 3: Advance further, no new triggers until 4.0f
   tracker_update(3.0f);