Parses an ISOBMFF file without loading it entirely into memory.

Walks the top-level boxes by seeking. Any box that is read into memory (a container, or a leaf smaller than the threshold) is parsed by re-running the in-memory ISOMedia.Parser over its exact bytes with offset: set, so its structure and absolute source_offsets match the eager parser. A leaf payload at or above :lazy_threshold becomes an ISOMedia.FileSlice and is never read.