Add the default-on Actions history setting and a grouping scope shared by ordinary engine and shell playback entry points. Define public regressions for whole-play Undo/Redo, the off setting, failure prefixes, nesting, no-op and repeated-play boundaries, and persistence. This is an unvalidated draft increment: dialog continuation, direct compound edits, Batch/droplet boundaries and the complete edge-case qualification remain unfinished.