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>

1 files changed, 81 insertions, 0 deletions

diff --git a/doc/TRACKER.md b/doc/TRACKER.md
index cb14755..f3a34a3 100644
--- a/doc/TRACKER.md
+++ b/doc/TRACKER.md
@@ -40,4 +40,85 @@ This generated code can be mixed with fixed code from the demo codebase
 itself (explosion predefined at a given time ,etc.)
 The baking is done at compile time, and the code will go in src/generated/
 
+## .track File Format
+
+### Timing System
+
+**Unit-less Timing Convention:**
+- All time values are **unit-less** (not beats or seconds)
+- Convention: **1 unit = 4 beats**
+- Conversion to seconds: `seconds = units * (4 / BPM) * 60`
+- At 120 BPM: 1 unit = 2 seconds
+
+This makes patterns independent of BPM - changing BPM only affects playback speed, not pattern structure.
+
+### File Structure
+
+```
+# Comments start with #
+
+BPM <tempo>              # Optional, defaults to 120 BPM
+
+SAMPLE <name>            # Define sample (asset or generated note)
+
+PATTERN <name> LENGTH <duration>   # Define pattern with unit-less duration
+  <unit_time>, <sample>, <volume>, <pan>   # Pattern events
+
+SCORE                    # Score section (pattern triggers)
+  <unit_time>, <pattern_name>
+```
+
+### Examples
+
+#### Simple 4-beat pattern (1 unit):
+```
+PATTERN kick_snare LENGTH 1.0
+  0.00, ASSET_KICK_1, 1.0, 0.0    # Start of pattern (beat 0)
+  0.25, ASSET_SNARE_1, 0.9, 0.0   # 1/4 through (beat 1)
+  0.50, ASSET_KICK_1, 1.0, 0.0    # 1/2 through (beat 2)
+  0.75, ASSET_SNARE_1, 0.9, 0.0   # 3/4 through (beat 3)
+```
+
+#### Score triggers:
+```
+SCORE
+  0.0, kick_snare      # Trigger at 0 seconds (120 BPM)
+  1.0, kick_snare      # Trigger at 2 seconds (1 unit = 2s at 120 BPM)
+  2.0, kick_snare      # Trigger at 4 seconds
+```
+
+#### Generated note:
+```
+SAMPLE NOTE_C4           # Automatically generates C4 note (261.63 Hz)
+PATTERN melody LENGTH 1.0
+  0.00, NOTE_C4, 0.8, 0.0
+  0.25, NOTE_E4, 0.7, 0.0
+  0.50, NOTE_G4, 0.8, 0.0
+```
+
+### Conversion Reference
+
+At 120 BPM (1 unit = 4 beats = 2 seconds):
+
+| Units | Beats | Seconds | Description |
+|-------|-------|---------|-------------|
+| 0.00  | 0     | 0.0     | Start       |
+| 0.25  | 1     | 0.5     | Quarter     |
+| 0.50  | 2     | 1.0     | Half        |
+| 0.75  | 3     | 1.5     | Three-quarter |
+| 1.00  | 4     | 2.0     | Full pattern |
+
+### Pattern Length
+
+- `LENGTH` parameter is optional, defaults to 1.0
+- Can be any value (0.5 for half-length, 2.0 for double-length, etc.)
+- Events must be within range `[0.0, LENGTH]`
+
+Example of half-length pattern:
+```
+PATTERN short_fill LENGTH 0.5   # 2 beats = 1 second at 120 BPM
+  0.00, ASSET_HIHAT, 0.7, 0.0
+  0.50, ASSET_HIHAT, 0.6, 0.0   # 0.50 * 0.5 = 1 beat into the pattern
+```
+