Undo after typing `---` should restore the dashes, not delete the divider

[virgile-dev]

The problem

Typing --- at the start of an empty block converts it to a divider. Pressing Ctrl/Cmd+Z right after does not bring back the three dashes: the divider stays.

Steps to reproduce

1. Place the cursor in an empty paragraph block.
2. Type ---.
3. The block becomes a divider.
4. Press Ctrl+Z (Cmd+Z on macOS).

Current behavior

The divider disappears. There is no way to get back to the literal --- text with undo.

Expected behavior

The first undo reverts the automatic conversion only and leaves --- as text in the paragraph, with the cursor after the last dash. A second undo then removes the typed characters as usual.

This is what Notion does, and it matches how users treat autoformatting in most editors: undo is the escape hatch when the editor guessed wrong.

Why it matters

Someone who wants three literal dashes (a separator in plain text, a markdown snippet, a table sketch) currently has no obvious way to keep them. They have to delete the divider and find a workaround to type the characters without triggering the rule. A conversion the user did not explicitly ask for should be reversible in one keystroke.

Environment
Tested on blocknotejs.org

[virgile-dev]

Markdown syntax asks for blank lines arround an horizontal line to render it.

Just like we have #[space] = H1 , maybe we should have ---[Enter] to get an horizontal line rendered ?
Also that would make us differ from Notion's convention, so might not be a good idea.