refactor(undo): de-Qt oakundo and wrap it in a pure C ABI

- 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

include/undo/error.h

@@ -0,0 +1,39 @@
+/***
+
+  Oak Video Editor - Non-Linear Video Editor
+  Copyright (C) 2026 Oak Team
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#ifndef OAK_EDITOR_UNDO_ERROR_H
+#define OAK_EDITOR_UNDO_ERROR_H
+
+/**
+ * @brief Status and error codes shared by all oakundo C API families.
+ *
+ * Return-code convention (mirrors engine/include/oakengine/init.h):
+ * 0 (OAKUNDO_OK) on success, a negative OAKUNDO_E_* error code on
+ * failure. String getters return the required buffer size in bytes
+ * (including the terminating NUL) as a non-negative value instead.
+ */
+#define OAKUNDO_OK 0 /**< Success. */
+#define OAKUNDO_E_INVALID (-1) /**< NULL handle or invalid argument. */
+#define OAKUNDO_E_STATE (-2) /**< Call not valid in the current state. */
+#define OAKUNDO_E_FAILED (-3) /**< The underlying operation failed. */
+#define OAKUNDO_E_NOT_FOUND (-4) /**< Index out of range / entry not found. */
+#define OAKUNDO_E_NOMEM (-5) /**< Allocation failed. */
+
+#endif //OAK_EDITOR_UNDO_ERROR_H