C++26: Standard Library Hardening Experiments

C++26 adds a standardized “hardened” mode for the standard library, where precondition violations such as out-of-bounds vector indexing trigger a contract-violation termination instead of undefined behaviour. The mode is enabled via implementation-defined macros—_GLIBCXX_ASSERTIONS for GCC, _LIBCPP_HARDENING_MODE for Clang, and _MSVC_STL_HARDENING for MSVC—while still preserving the distinct semantics of operator[] and .at(). The feature follows the new Contracts model and is being implemented according to proposals P3471, P3697, and P3878.