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

pyproject.toml

@@ -8,7 +8,6 @@ name = "astrai"
 readme = "README.md"
 requires-python = ">=3.12"
 dependencies = [
-    "h5py==3.15.1",
     "numpy==2.4.4",
     "torch==2.11.0",
     "tokenizers==0.21.4",