Session::grouped saved only the undo stack's length, so a failed group could drop the entries it
added but not bring back the ones an edit.undo inside it popped. It now keeps each document's undo
entries on entry, as it already did for redo, and restores them when the group fails.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Co-Authored-By: GPT-6.1 Sol <noreply@openai.com>
Co-Authored-By: GPT-6 Astra <noreply@openai.com>