✨ template: add Meson C++ project template

[lalmei]

Summary

• Add a separate C++ Copier template modeled on the Spinach project structure.
• Wire the cpp preset and create subcommand into the CLI.
• Add docs and tests for the new template path.

Verification

• make check
• make test
• generated a sample C++ project and ran make setup, make check, make test, and make docs-check

---

This is part 3 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