applyPlainTextToRichText

Fold a plain-text edit back into a rich-text document.

The plain-text editing surface (the <textarea> the DOM overlay falls back to when no TipTap editor is installed) can only express text, not marks. When the text did not actually change, the original document is returned unchanged so opening and closing the editor never silently strips formatting. When it did change, the document is rebuilt from the new text — the honest outcome for an editor that cannot represent the marks it would have to preserve.

SEMANTICS-ASSUMED: mocanvas owns this seam (it exists because TipTap is optional here), so there is no upstream behaviour to match. "Keep formatting on a no-op, drop it on a real plain-text edit" is the conservative choice: it never loses formatting the user did not touch, and never claims to preserve marks it cannot place.