Function decode_sparse_cache 

pub fn decode_sparse_cache(
    buf: &[u8],
) -> Result<(Hash, Hash, u64, Vec<u8>), SparseWireError>

Decode the on-disk cache payload. Returns the bitmap root, filter hash, leaf count, and bitmap bytes — the caller reconstructs the SparseManifest if needed (the tree_hash field comes from the filename, not from the file body).

Errors

Same shape as decode_sparse_response.