Make error code types nodiscard

style.md

@@ -344,9 +344,10 @@ arena_allocator.reset();  // Reset arena memory
 - **Error messages are human-readable only** - never parse message strings
 - **Consistent error boundaries** - each component defines what it can/cannot recover from
 - **Interface precondition violations are undefined behavior** - acceptable to skip checks for performance in hot paths
+- **Error code types must be nodiscard** - mark error code enums with `[[nodiscard]]` to prevent silent failures
 
 ```cpp
-enum class ParseResult { Success, InvalidJson, MissingField };
+enum class [[nodiscard]] ParseResult { Success, InvalidJson, MissingField };
 
 // System failure - abort immediately
 void* memory = std::malloc(size);