- strip QAbstractItemModel/QAction/signals-slots from UndoStack (UI
concerns belong to the app layer), index_changed becomes a
std::function callback
- decouple UndoCommand from Project via an optional modified-flag
callback pair
- add pure C ABI in include/undo + src/undo/c_api (oakundo_ prefix,
vtable-based command wrapper, OAKUNDO_E_* error codes)
- fix three engine-side defects: jump(0) infinite loop,
MultiUndoCommand child leak, push_pre_executed not undoable
- add gtest suites (22 cases) and a standalone build driver