std::fmt::Display for user-facing string representations

Implementing Display lets your types be formatted with {} in format strings. It's for user-facing output, unlike Debug which is for developers. I implement Display for error types, domain models, and anything that might be printed. The trait requires a fmt method that writes to a formatter. For simple cases, use write!(f, "...", args). For complex formatting, build the string incrementally. Display is also required for converting to strings via .to_string(). I derive or implement it on most public types to improve ergonomics. It's a small effort that makes your API feel polished.