Function mkfs_erofs 

pub fn mkfs_erofs<ObjectID: FsVerityHashValue>(
    fs: &FileSystem<ObjectID>,
) -> Box<[u8]>

Creates an EROFS filesystem image from a composefs tree

This function performs a two-pass generation:

1. First pass determines the layout and sizes of all structures
2. Second pass writes the actual image data

Returns the complete EROFS image as a byte array.