Function part_path 

pub fn part_path(final_path: &str, part_idx: usize) -> String

Returns the temporary spill-file path for one shard-local intermediate.

Stable zero-padded numbering keeps shard files easy to scan by eye and avoids mixing two historical naming schemes (.shard_N vs fsj_shard_N). The input must be the logical merged artifact name or stem without a pre-existing .tmp suffix; shard files are always named <merged>.part_XXXX.tmp so every parallel stage uses the same convention.