Add PEP 561 type stubs for chiavdf

[emlowe]

Summary

• Package chiavdf as a proper PEP 561 module with py.typed and __init__.pyi
• Move the pybind11 extension to private chiavdf._chiavdf and re-export the public API from chiavdf/__init__.py (import path unchanged for callers)
• Add stubtest to the Ubuntu check job (build extension, then validate stubs against runtime)
• Avoid cibuildwheel importing the source tree during wheel tests by temporarily renaming chiavdf/ (same pattern as other Chia native wheels)

Test plan

• CI Check - Ubuntu 3.12 Intel passes (flake8, mypy, stubtest)
• CI cibuildwheel matrix passes on Linux/macOS/Windows
• After release, bump chiavdf in chia-blockchain and remove [mypy-chiavdf.*] ignore_missing_imports

Made with Cursor

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Note

Low Risk
Changes are packaging, typing, and CI only; VDF logic is unchanged aside from renaming the extension module, with stubtest intended to catch API mismatches.

Overview
This PR makes chiavdf a PEP 561–typed package while keeping the public import path the same (from chiavdf import …).

The pybind11 extension is built as chiavdf._chiavdf instead of a top-level extension module; chiavdf/__init__.py re-exports the VDF API, and __init__.pyi plus py.typed document types for mypy and other checkers. setup.py / CMake / fastvdf.cpp are updated so the native target is _chiavdf.

CI on Ubuntu gains cmake/GMP install deps, extends flake8 and mypy to the chiavdf package, and runs stubtest after pip install .. cibuildwheel temporarily renames the source chiavdf/ tree during wheel tests so pytest uses the built wheel, not the checkout.

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