Function write_tree 

pub fn write_tree(
    paths: &OutputPaths,
    manifest_toml: &str,
    artifact_path: &Path,
    sha256_hex: &str,
    signature: &[u8],
    dry_run: bool,
) -> Result<Vec<u8>>

Write the full tree atomically-ish: create dirs, then write each file. Returns the manifest bytes written (for signing). When dry_run is true, no filesystem writes occur and an empty Vec is returned.