Function collect_objects_from_fs 

fn collect_objects_from_fs(
    fs: &FileSystem<Sha256HashValue>,
) -> HashSet<Sha256HashValue>

Collect all external object IDs from a parsed filesystem.

Iterates the leaves table directly — each RegularFile::External entry is a unique content-addressed object. Because erofs_to_filesystem deduplicates hard-linked inodes into a single leaf, each object appears exactly once even if it is referenced by multiple paths.