feat(test_demo): Add beat_number column to fine-grained peak log

Adds beat_number as 4th column in fine-grained logging mode to enable
easy correlation between frame-level data and beat boundaries.

File format change:
- Before: frame_number clock_time raw_peak
- After:  frame_number clock_time raw_peak beat_number

Benefits:
- Correlate frame-level peaks with specific beats
- Filter or group data by beat in analysis scripts
- Easier comparison between beat-aligned and fine-grained logs
- Identify which frames belong to each beat interval

Example output:
  0 0.000000 0.850000 0
  1 0.016667 0.845231 0
  ...
  30 0.500000 0.720000 1
  31 0.516667 0.715234 1

This allows filtering like: awk '$4 == 0' peaks_fine.txt
to extract all frames from beat 0.

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

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