refactor: remove H5 storage backend in favor of mmap bin

- Remove H5Store, H5Writer, save_h5/load_h5 and h5py dependency
- MmapStore (bin) is the sole pre-tokenized storage backend
- Move setup_logging after imports to fix E402 in __init__.py
- Clean up unused imports across test files
- Move inline test imports to file top

tests/trainer/test_grpo_strategy.py

@@ -3,7 +3,7 @@ import torch
 
 from astrai.model.transformer import AutoRegressiveLM
 from astrai.trainer.strategy import GRPOStrategy
-from tests.helpers import FakeExecutor, make_frozen, make_model, make_rollout_config
+from tests.helpers import FakeExecutor, make_frozen, make_model
 
 
 def _make_batch(