The `if undoable { mark_document_changed() }` added to the live-sync glue was
redundant: `replace_document_with_undo` already bumps the content revision via
`history_push_past` -> `mark_document_changed`, so an undoable external apply is
dirty by construction. Remove the glue's manual bump (and its now-wrong comment);
the post-apply pair capture + `note_synced` stay put, since that pair already
carries the history bump.
Retarget the editor-core test to lock the real invariant:
`replace_document_with_undo` ALONE leaves the state dirty (revision != saved),
while plain `replace_document` stays clean -- so a future refactor of
`history_push_past` can't silently break external-apply dirtiness.
2fd88b601e