- Create AddAnnotationCommand (Command/Handler/Validator merged in single file)
- Use primary constructors for handler (IPdfProcessor dependency)
- FluentValidation rules: stream required, pageNumber > 0, content for FreeText/StickyNote
- Validate textMarkupStyle required for TextMarkup annotations
- Validate color format (6-digit hex) and rectangle coordinates (X2>X1, Y2>Y1)
