Move JSONL serialization from standalone functions to model methods
to mirror the from_jsonl() pattern:
- Add Binary.to_jsonl() method
- Add Process.to_jsonl() method
- Add ArchiveResult.to_jsonl() method
- Add Snapshot.to_jsonl() method
- Update write_index_jsonl() to use model methods
- Update jsonl.py functions to be thin wrappers