refactor(audio): Convert tracker to unit-less timing system

Changes tracker timing from beat-based to unit-less system to separate
musical structure from BPM-dependent playback speed.

TIMING CONVENTION:
- 1 unit = 4 beats (by convention)
- Conversion: seconds = units * (4 / BPM) * 60
- At 120 BPM: 1 unit = 2 seconds

BENEFITS:
- Pattern structure independent of BPM
- BPM changes only affect playback speed, not structure
- Easier pattern composition (0.00-1.00 for typical 4-beat pattern)
- Fixes issue where patterns played for 2s instead of expected duration

DATA STRUCTURES (tracker.h):
- TrackerEvent.beat → TrackerEvent.unit_time
- TrackerPattern.num_beats → TrackerPattern.unit_length
- TrackerPatternTrigger.time_sec → TrackerPatternTrigger.unit_time

RUNTIME (tracker.cc):
- Added BEATS_PER_UNIT constant (4.0)
- Convert units to seconds at playback time using BPM
- Pattern remains active for full unit_length duration
- Fixed premature pattern deactivation bug

COMPILER (tracker_compiler.cc):
- Parse LENGTH parameter from PATTERN lines (defaults to 1.0)
- Parse unit_time instead of beat values
- Generate code with unit-less timing

ASSETS:
- test_demo.track: converted to unit-less (8 score triggers)
- music.track: converted to unit-less (all patterns)
- Events: beat/4 conversion (e.g., beat 2.0 → unit 0.50)
- Score: seconds/unit_duration (e.g., 4s → 2.0 units at 120 BPM)

VISUALIZER (track_visualizer/index.html):
- Parse LENGTH parameter and BPM directive
- Convert unit-less time to seconds for rendering
- Update tick positioning to use unit_time
- Display correct pattern durations

DOCUMENTATION (doc/TRACKER.md):
- Added complete .track format specification
- Timing conversion reference table
- Examples with unit-less timing
- Pattern LENGTH parameter documentation

FILES MODIFIED:
- src/audio/tracker.{h,cc} (data structures + runtime conversion)
- tools/tracker_compiler.cc (parser + code generation)
- assets/{test_demo,music}.track (converted to unit-less)
- tools/track_visualizer/index.html (BPM-aware rendering)
- doc/TRACKER.md (format documentation)
- convert_track.py (conversion utility script)

TEST RESULTS:
- test_demo builds and runs correctly
- demo64k builds successfully
- Generated code verified (unit-less values in music_data.cc)

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

2 files changed, 17 insertions, 14 deletions

diff --git a/src/audio/tracker.cc b/src/audio/tracker.cc
index 7ad5a67..9ae772e 100644
--- a/src/audio/tracker.cc
+++ b/src/audio/tracker.cc
@@ -212,19 +212,24 @@ static void trigger_note_event(const TrackerEvent& event) {
 }
 
 void tracker_update(float music_time_sec) {
+  // Unit-less timing: 1 unit = 4 beats (by convention)
+  const float BEATS_PER_UNIT = 4.0f;
+  const float unit_duration_sec = (BEATS_PER_UNIT / g_tracker_score.bpm) * 60.0f;
+
   // Step 1: Process new pattern triggers
   while (g_last_trigger_idx < g_tracker_score.num_triggers) {
     const TrackerPatternTrigger& trigger =
         g_tracker_score.triggers[g_last_trigger_idx];
 
-    if (trigger.time_sec > music_time_sec)
+    const float trigger_time_sec = trigger.unit_time * unit_duration_sec;
+    if (trigger_time_sec > music_time_sec)
       break;
 
     // Add this pattern to active patterns list
     const int slot = get_free_pattern_slot();
     if (slot != -1) {
       g_active_patterns[slot].pattern_id = trigger.pattern_id;
-      g_active_patterns[slot].start_music_time = trigger.time_sec;
+      g_active_patterns[slot].start_music_time = trigger_time_sec;
       g_active_patterns[slot].next_event_idx = 0;
       g_active_patterns[slot].active = true;
     }
@@ -233,8 +238,6 @@ void tracker_update(float music_time_sec) {
   }
 
   // Step 2: Update all active patterns and trigger individual events
-  const float beat_duration = 60.0f / g_tracker_score.bpm;
-
   for (int i = 0; i < MAX_SPECTROGRAMS; ++i) {
     if (!g_active_patterns[i].active)
       continue;
@@ -242,15 +245,15 @@ void tracker_update(float music_time_sec) {
     ActivePattern& active = g_active_patterns[i];
     const TrackerPattern& pattern = g_tracker_patterns[active.pattern_id];
 
-    // Calculate elapsed beats since pattern started
+    // Calculate elapsed unit-less time since pattern started
     const float elapsed_music_time = music_time_sec - active.start_music_time;
-    const float elapsed_beats = elapsed_music_time / beat_duration;
+    const float elapsed_units = elapsed_music_time / unit_duration_sec;
 
-    // Trigger all events that have passed their beat time
+    // Trigger all events that have passed their unit time
     while (active.next_event_idx < pattern.num_events) {
       const TrackerEvent& event = pattern.events[active.next_event_idx];
 
-      if (event.beat > elapsed_beats)
+      if (event.unit_time > elapsed_units)
         break; // This event hasn't reached its time yet
 
       // Trigger this event as an individual voice
@@ -259,8 +262,8 @@ void tracker_update(float music_time_sec) {
       active.next_event_idx++;
     }
 
-    // If all events have been triggered, mark pattern as complete
-    if (active.next_event_idx >= pattern.num_events) {
+    // Pattern remains active until full duration elapses
+    if (elapsed_units >= pattern.unit_length) {
       active.active = false;
     }
   }
diff --git a/src/audio/tracker.h b/src/audio/tracker.h
index 336f77f..4cd011b 100644
--- a/src/audio/tracker.h
+++ b/src/audio/tracker.h
@@ -8,7 +8,7 @@
 #include <cstdint>
 
 struct TrackerEvent {
-  float beat;
+  float unit_time;  // Unit-less time within pattern (0.0 to pattern.unit_length)
   uint16_t sample_id;
   float volume;
   float pan;
@@ -17,11 +17,11 @@ struct TrackerEvent {
 struct TrackerPattern {
   const TrackerEvent* events;
   uint32_t num_events;
-  float num_beats;
+  float unit_length;  // Pattern duration in units (typically 1.0 for 4-beat patterns)
 };
 
 struct TrackerPatternTrigger {
-  float time_sec;
+  float unit_time;  // Unit-less time when pattern triggers
   uint16_t pattern_id;
   // Modifiers could be added here
 };
@@ -29,7 +29,7 @@ struct TrackerPatternTrigger {
 struct TrackerScore {
   const TrackerPatternTrigger* triggers;
   uint32_t num_triggers;
-  float bpm;
+  float bpm;  // BPM is used only for playback scaling (1 unit = 4 beats)
 };
 
 // Global music data generated by tracker_compiler