Fix Windows in-place rebuild locking its existing extension

[drebbe-intrepid]

Windows in-place rebuilds imported the existing extension twice before attempting to replace it: generated-structure validation and setuptools dynamic version lookup. Run validation in a checked subprocess that exits before copying the extension, and read the generated version literal directly.

Adds six regression cases for child-process isolation, import failures, regeneration, and metadata lookup, plus two consecutive forced in-place builds in Windows CI.

Validation (Windows x64, CPython 3.14.5):

• Reproduced original second-build failure: Access is denied replacing the existing .pyd.
• Two consecutive python setup.py build_ext --inplace --force commands pass after the fix.
• python setup.py build_py, then PYTHONPATH=gen python -m pytest tests -q: 36 passed.
• Independent critic /root/critic agrees with final five-file diff; builder agrees. Critic independently ran all six regression tests and inspected build logs.

Fixes #249