C++26: Standard Library Hardening Experiments

C++26 introduces "hardened" standard library implementations that detect programming errors like out-of-bounds access in `std::vector::operator[]` by triggering contract violations with terminating semantics, rather than allowing undefined behavior. This differs from `.at()` by maintaining the same interface and performance characteristics while providing safety checks that can be enabled or disabled depending on the implementation. The hardening feature uses C++26's new Contracts model and runs at runtime, offering a middle ground between the unsafe but fast unchecked access and the exception-based safety of `.at()`.