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/dataset/__init__.py

@@ -5,10 +5,13 @@ from astrai.dataset.dataset import (
 from astrai.dataset.sampler import ResumableDistributedSampler
 from astrai.dataset.storage import (
     H5Store,
+    JsonlStore,
     MmapStore,
     Store,
     StoreFactory,
     detect_format,
+)
+from astrai.serialization import (
     load_bin,
     load_h5,
     save_bin,
@@ -22,6 +25,7 @@ __all__ = [
     "StoreFactory",
     "H5Store",
     "MmapStore",
+    "JsonlStore",
     "detect_format",
     "save_h5",
     "load_h5",