pub fn merkle_default_cache_path() -> Option<PathBuf>
Default Merkle index location: $XDG_CACHE_HOME/keyhog/merkle.idx or ~/.cache/keyhog/merkle.idx on Linux, ~/Library/Caches/keyhog/... on macOS.
$XDG_CACHE_HOME/keyhog/merkle.idx
~/.cache/keyhog/merkle.idx
~/Library/Caches/keyhog/...