Guidance for writing, reading, updating, building, and testing C++ code for the 64-bit RISC OS build environment. Use whenever Codex works on RISC OS C++ command projects, `cc/` sources, C++ headers in `h/`, `riscos64-c++`, `riscos-project --type c++command`, `${C++LIB}`, or C++ interop with C and RISC OS APIs.
