Error Handling and Utilities

Error handling and runtime utilities: turning "maybe nothing," "closed polymorphism," and "value or error" into types with optional/variant/expected, the cost of function objects and std::function, the error code system error_code, C++23's standardized stack capture with stacktrace, and compile-time code location via source_location.

optional: Making "Maybe Nothing" a Type→variant: Type-Safe Unions and visit→any: Holding Any Type→expected: Value or Error (C++23)→functional: The Cost of std::function→error_code: Error Code Systems and Custom category→stacktrace: C++23 Stack Capture→source_location: Compile-Time Code Location→