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

src/undo/c_api/commandhandle.h

@@ -0,0 +1,41 @@
+/***
+
+  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_UNDO_COMMANDHANDLE_H
+#define OAK_UNDO_COMMANDHANDLE_H
+
+#include "undo/undocommand.h"
+
+#include "../src/undocommand.h"
+
+/**
+ * @brief Internal layout of the OakUndoCommand handle, shared between the
+ * c_api translation units.
+ *
+ * `owned` is true for handles created by oakundo_command_init() /
+ * oakundo_command_init_multi() and false for borrowed wrappers handed out
+ * by oakundo_command_multi_child().
+ */
+struct OakUndoCommand {
+	olive::UndoCommand *command;
+	bool owned;
+};
+
+#endif // OAK_UNDO_COMMANDHANDLE_H