build.rs cleanup

[jorenham]

this should make it easier to more custom c++ wrapper functions

[Copilot]

Pull Request Overview

This PR restructures the build.rs file to improve organization and maintainability of C++ wrapper function generation. The changes move inline string literals to constants and refactor helper functions for better modularity.

Key changes:

• Extract inline C++ code strings to named constants (DECL_, IMPL_)
• Refactor formatting functions to use a more functional programming style
• Consolidate allowlist generation into a dedicated function

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}