chore(gui3): synchronize backend with main

[kpoineal]

Spec Driven Development

• This is a backend synchronization that does not change GUI3 scope or UX and does not need a WG/RFC.

Summary

Bring the current main C++/backend implementation into GUI3_squashed while preserving the GUI3 frontend. This includes the required CMake version helper and keeps the GUI3 tool-definition path compatible without changing frontend files.

Scope

• This PR addresses one clear change.
• I reviewed the full diff myself before submitting.
• I removed unrelated local changes.
• I kept refactoring separate unless required for this synchronization.

Testing

• The code change has been locally tested.

Testing details:

• Pre-commit hooks passed.
• cmake --preset windows
• cmake --build --preset windows --target cpp-ci-tests --parallel 4

Documentation

• Documentation is not affected by this change.

Breaking Changes

• This PR does not introduce breaking changes.

AI-assisted contribution

Prepared with GitHub Copilot and reviewed against the full diff to ensure no GUI3/frontend paths are included.

[kpoineal]

@jeremyfowers

Can you review and lmk?

[jeremyfowers]

@kpoineal please get the tests passing first.