feat: add JSONL dataset store with on-the-fly tokenization

- Add JsonlStore registered under "jsonl" in astrai/dataset/storage.py
- Reuse PipelineConfig schema for JSONL dataset configuration
- Update detect_format to recognize JSONL directories and files
- Move save_h5/load_h5/save_bin/load_bin to astrai/serialization
- Split astrai/serialization.py into checkpoint/dataset submodules
- Add tests for JSONL detection, seq/SFT stores, and config roundtrip

astrai/preprocessing/writer.py

@@ -14,8 +14,8 @@ from typing import Dict, List
 
 import torch
 
-from astrai.dataset.storage import save_bin, save_h5
 from astrai.factory import BaseFactory
+from astrai.serialization import save_bin, save_h5
 
 logger = logging.getLogger(__name__)