Export well-known errors and support errors.Is

[kpiljoong]

Current parse/validation functions likely return ad-hoc errors (string messages).

Proposal:

• Define package-level error variables, e.g.:

  var (
    ErrInvalidPrefix    = errors.New("orderlyid: invalid prefix")
    ErrInvalidChecksum  = errors.New("orderlyid: invalid checksum")
    ErrInvalidFormat    = errors.New("orderlyid: invalid format")
  )
  

• Use fmt.Errorf("%w: details", ErrInvalidPrefix) where needed.

Acceptance criteria:

• Callers can use errors.Is(err, ErrInvalidChecksum) etc.
• Documentations lists possible error values.