cuikp/AvRichTextBox — reverse-engineered prompt

I want to build a reusable RichTextBox control for Avalonia, plus a small demo app that shows it working.

The control should let people type and edit rich text with multiple paragraphs, select text across paragraphs with the mouse or keyboard, and use common shortcuts like Ctrl+B for bold, Ctrl+I for italic, Ctrl+U for underline, Ctrl+Z for undo, and Ctrl+A for select all. It should support inline formatted text, images inside the document, copy and paste of rich text, and Chinese or Japanese IME input.

Please make it feel similar to the WPF RichTextBox, with a document model that can be edited from code, including selections and text ranges that can apply formatting. It should be able to save and load content as Xaml, XamlPackage for images, and basic docx, rtf, and html with common formatting like fonts, colors, highlights, paragraph alignment, borders, and images.

Add a debug only panel that can show selection and inline details. Look up current Avalonia docs online if needed.