Error Handling and Utilities

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

STARTFINISHoptional: Making "Maybe Nothing" a Type→STARTFINISHvariant: Type-Safe Unions and visit→STARTFINISHany: Holds Any Type→STARTFINISHexpected: Value or Error (C++23)→STARTFINISHfunctional: The Cost of std::function→STARTFINISHerror_code: The Error Code System and Custom Categories→STARTFINISHstacktrace: C++23 Call Stack Capture→STARTFINISHsource_location: Compile-Time Code Location→