✨ template: add C++ Python bindings

[lalmei]

Summary:

• Add pybind11-backed Python bindings to the bundled C++ template.
• Wire the binding build through Copier defaults, Meson options, and generated project dependencies.
• Add generated binding smoke tests plus repoman template assertions and docs.

Validation:

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

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This is part 1 of 4 in a stack made with GitButler:

•  4  ✨ template: add C++ Python bindings #109 👈
•  3  ✨ template: add LaTeX paper scaffold #108
•  2  ✨ template: add Meson C++ project template #107
•  1  🚚 docs: switch generated docs tooling to ProperDocs #106