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>

1 files changed, 9 insertions, 5 deletions

diff --git a/test_demo_README.md b/test_demo_README.md
index b8abfc2..bbce023 100644
--- a/test_demo_README.md
+++ b/test_demo_README.md
@@ -151,12 +151,15 @@ Logs at every frame (approximately 960 samples for 16 seconds at 60 Hz):
 # Mode: fine (per-frame)
 # To plot with gnuplot:
 #   gnuplot -p -e "set xlabel 'Time (s)'; set ylabel 'Peak'; plot 'peaks_fine.txt' using 2:3 with lines title 'Raw Peak'"
-# Columns: frame_number clock_time raw_peak
+# Columns: frame_number clock_time raw_peak beat_number
 #
-0 0.000000 0.850000
-1 0.016667 0.845231
-2 0.033333 0.823445
-3 0.050000 0.802891
+0 0.000000 0.850000 0
+1 0.016667 0.845231 0
+2 0.033333 0.823445 0
+3 0.050000 0.802891 0
+...
+30 0.500000 0.720000 1
+31 0.516667 0.715234 1
 ...
 ```
 
@@ -164,6 +167,7 @@ Logs at every frame (approximately 960 samples for 16 seconds at 60 Hz):
 1. **frame_number**: Frame index (0, 1, 2, ...)
 2. **clock_time**: Physical time in seconds (millisecond precision)
 3. **raw_peak**: Audio peak value (0.0-1.0+)
+4. **beat_number**: Corresponding beat index (for correlation with beat-aligned mode)
 
 **Use Cases:**