Extend native C++

Build requirements are C++20, CMake 3.24+, Qt/PySide/Shiboken 6.8.x, Slang, and Python 3.11–3.13. SDK locations come from CMAKE_PREFIX_PATH and SLANG_ROOT.

Choose the layer by ownership, not convenience: core values in core/, compiler integration in slang/, QRhi consumption in rendering/, reusable widgets in ui/, and tool policy in modes/. See Native source layout.