✨ template: expose C++ binding version

[lalmei]

Summary:

• Add version to generated pybind11 modules.
• Document a quick PYTHONPATH=build import command in generated C++ README files.
• Extend generated and repoman template tests to cover the version attribute.

Validation:

• make check
• make check-docs
• uv run pytest -c=config/pytest.ini --no-cov tests/test_template/test_cpp_template.py
• Fresh generated C++ project: make setup, make test, make format-check, and direct version import check

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This is part 2 of 2 in a stack made with GitButler:

•  2  ✅ tests: strengthen C++ binding smoke tests #111
•  1  ✨ template: expose C++ binding version #110 👈