chore: add py.typed marker for PEP 561 typing support

[speckhard]

Summary

Add an empty py.typed marker file so downstream type checkers (mypy, pyright, pyre) actually pick up the hand-maintained .pyi stubs shipped alongside the package.

PEP 561 requires this marker for "inline" packages — packages that ship their stubs in the same directory as the runtime code, which is what atompack does. Without the marker, mypy / pyright treat every imported atompack symbol as Any, even though the stubs are correct.

Why no pyproject.toml change

Maturin ships everything under python-source = "python" by default. The new file is in atompack-py/python/atompack/py.typed, so it lands in the wheel without any [tool.maturin] include change. The three .pyi stubs already in that directory ship the same way today — confirmed by inspecting the published atompack-db==0.2.1 wheel.

Verification

Maturin packaging path. Inspected the current PyPI wheel:

$ unzip -l atompack_db-0.2.1-cp311-cp311-macosx_11_0_arm64.whl | grep atompack/
   1516  atompack/__init__.py
  20506  atompack/__init__.pyi
2074560  atompack/_atompack_rs.cpython-311-darwin.so
  13046  atompack/_atompack_rs.pyi
  21944  atompack/ase_bridge.py
  15161  atompack/hub.py
   4492  atompack/hub.pyi

The three .pyi stubs ship today via python-source = "python". A sibling py.typed rides the exact same path.

End-to-end mypy check. Fresh Python 3.11 venv with the current PyPI wheel installed.

Before adding py.typed:

$ mypy -c "import atompack; reveal_type(atompack.Database)"
<string>:1: error: Module has no attribute "Database"  [attr-defined]
<string>:1: note: Revealed type is "Any"
Found 1 error in 1 file (checked 1 source file)

After touch site-packages/atompack/py.typed:

$ mypy -c "import atompack; reveal_type(atompack.Database)"
<string>:1: note: Revealed type is "def (path: str, compression: str =, level: int =, overwrite: bool =) -> atompack.Database"
Success: no issues found in 1 source file

Confirms the marker is exactly what unlocks the existing stubs for downstream type checkers.

References

• PEP 561 — https://peps.python.org/pep-0561/