The bg_avg>1 path kept all n_synth frames and only added an averaged
background, so AVG4 and AVG8 produced the same frame count and 1/fps
timestamps. Now output frame i is the mean of bg_avg consecutive frames of
BOTH the scaled synthetic target AND the real background (non-overlapping
groups): the sequence is decimated by N (n_synth/N frames), the moving target
is motion-averaged like a real pyramid level, and each timestamp is the last
real frame of its group (N/fps increments).
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>