Function collect_file_hashes 

pub fn collect_file_hashes(
    repo_root: &Path,
    commit: &str,
    prefix: &str,
) -> Result<HashMap<String, String>, Box<dyn Error>>

Collect file hashes for a single git commit.

Returns a map of (file_path → blob_hash) for all .rs files under prefix. Uses git ls-tree which gives us the blob hash — this IS content addressing.