Function write_archive_from_directory 

pub fn write_archive_from_directory<W: Write + Seek>(
    source_dir: &Path,
    writer: W,
    compress: &str,
) -> Result<FxarExportStats>

Write an FXAR v2 archive directly from a directory (no VersionStore required).

Walks source_dir and archives all files with bare relative paths in the manifest (no {snapshot}/tree/ prefix). This is the “just archive this directory” mode – like tar -cf archive.fxar /source.