C++26 Contracts Make API Rules Explicit

C++26 Contracts are a meaningful upgrade to API clarity, but they are not a substitute for input validation or guaranteed runtime safety.

• –Preconditions and postconditions make caller obligations and function guarantees visible in declarations.
• –Contracts are associated with functions but do not affect function types or overload resolution.
• –Four semantics—ignore, observe, enforce, and quick-enforce—give implementations flexibility, but that flexibility can still allow invalid execution to continue.
• –GCC 16 provides the first practical implementation, with support still described as experimental by [GCC](https://gcc.gnu.org/projects/cxx-status.html).
• –Developers should keep predicates side-effect-free and use ordinary control flow for errors the program is expected to handle.