Function build_pack_with_parallel_threads 

pub fn build_pack_with_parallel_threads(
    odb: &Odb,
    wants: &[ObjectId],
    haves: &[ObjectId],
    opts: &PackBuildOptions,
    search_threads: Option<usize>,
    write_threads: Option<usize>,
) -> Result<Vec<u8>>

Like build_pack with explicit delta-search and pack-write worker overrides.