v0.3.0

Added

• AppCode — stable machine-readable error code (part of the wire contract).
• ErrorResponse.code, ErrorResponse.retry, ErrorResponse.www_authenticate fields.
• Axum/Actix integrations now set Retry-After and WWW-Authenticate headers when applicable.

Changed (breaking)

• ErrorResponse::new now requires (status: u16, code: AppCode, message: impl Into<String>).

Migration

• Replace ErrorResponse::new(status, "msg") with
  ErrorResponse::new(status, AppCode::<Variant>, "msg").
• Optionally use .with_retry_after_secs(...) and/or .with_www_authenticate(...)
  to populate the new fields.