Function downsamplePeaks

Downsamples a raw mono sample buffer into barCount per-bucket amplitudes and normalizes the result to 0..1 (the loudest bucket becomes 1).

The samples are partitioned into barCount contiguous, (near-)equal buckets in index order; each bucket is reduced to one amplitude via aggregation:

• 'max-abs' (default) — the maximum absolute sample in the bucket (crisp peaks),
• 'rms' — the root-mean-square of the bucket (smoother, energy-weighted).

Edge cases:

• empty / null input → an array of barCount zeros,
• fewer samples than bars → every sample still lands in a bucket; trailing empty buckets are 0 (no out-of-bounds reads),
• a silent or constant-zero buffer → all zeros (normalization is skipped when the peak is 0, so we never divide by zero).