Add _core.pyi type stubs and py.typed marker

[dmartmillan]

Description
Create openvariant/_core.pyi with complete type annotations for all classes and functions exposed by the Rust extension. Add openvariant/py.typed (PEP 561).

Files

• openvariant/_core.pyi — NEW
• openvariant/py.typed — NEW

Definition of Done

• Stubs cover: Annotation, VariantReader, cat_task, count_task, group_by_task, find_files
• mypy openvariant/ runs clean (no missing stubs errors)
• py.typed present for downstream type-checking support

Documentation needed: Update README.md to mention PEP 561 compatibility.

Tests needed:

• Add mypy to CI: mypy openvariant/ --ignore-missing-imports must exit 0