Function calculate_adaptive_chunk_size 

pub fn calculate_adaptive_chunk_size(file_size: u64) -> u64

Calculate chunk size for Merkle tree construction.

For files that fit in xattr (<=~115MB): uses adaptive sizing to keep <=1800 leaves. For large files (>256MB): uses 512KB chunks stored in per-target Merkle index, giving 512KB resync granularity instead of the 55MB+ that xattr limits force.