feat: Add Gantt chart visualization to seq_compiler

Implements ASCII Gantt chart generation for timeline debugging and visualization.

## New Feature
- Added --gantt=<output.txt> flag to seq_compiler
- Generates visual timeline showing sequences and effects on time axis
- Displays sequence priority, effect priority, and time ranges
- Shows explicit sequence end times with [END=...] markers
- Detects and warns about invalid time ranges (end < start)

## Usage
```bash
./build/seq_compiler assets/demo.seq src/generated/timeline.cc --gantt=timeline.txt
```

## Chart Format
- Time axis in seconds with 5-second markers
- Sequences shown as solid bars (█)
- Effects shown as shaded bars (▓) with sequence background (·)
- Labels include start/end times and priorities
- Legend and documentation at chart end

## Example Output
```
Time (s): 0    5    10   15   20   25   30
          |----|----|----|----|----|----|----|

SEQ@0s [pri=0]
          ████████████████████████████████  (0-30s)
  FlashEffect [pri=4]
          ▓▓··························  (0-1s)
  HeptagonEffect [pri=0]
          ▓▓▓▓▓▓▓▓▓▓▓▓················  (0-10s)
```

## Benefits
- Visualize sequence overlap and layering
- Identify timing conflicts and gaps
- Verify effect priorities render in correct order
- Debug invalid time ranges
- Plan demo choreography visually

## Files Changed
- tools/seq_compiler.cc: Added generate_gantt_chart() function
- assets/demo.seq: Added usage documentation
- .gitignore: Exclude generated demo_timeline.txt

This debugging tool significantly improves timeline development workflow
by providing visual feedback on sequence and effect timing.

0 files changed, 0 insertions, 0 deletions