Conflicts:
- content_ui.rs: main's added-text editor (alignment, character
spacing) kept whole; the box takes input_id() instead of an id_salt,
and its Undo history is reset per document by forget_typing_before.
- tests/it/editing.rs: both sides' new tests kept.
#3546's TextDraft literals (editing.rs, edit_text_ui.rs tests) take
typing_from: None.