refactor(undo): switch oakundo to refcounted value handles, migrate consumers

- OakUndoCommand/OakUndoStack become neutral by-value handles
  {ctx, addref, release, abi_version}; a handle is a shared_ptr
  equivalent at the ABI level, internals untouched (box owns/observes
  flag; containers adopt, boxes created by factories own)
- oaknode command factories and undoable variants return/write value
  handles; timeline command classes hold value handles; task import
  take_command returns a value handle
- tests updated everywhere; suites green: oakundo 22, oaktimeline 117,
  oaktask 106, oaknode 96, oakrender 44, oakcodec 18, oakcommon 193,
  oakaudio 36

include/task/project.h

@@ -57,7 +57,7 @@ OakTaskTask *oaktask_create_project_import(OakNodeFolder *folder,
 										   int url_count);
 
 /** @brief Take the import's undo command (ownership transfer). */
-OakUndoCommand *oaktask_import_take_command(OakTaskTask *t);
+OakUndoCommand oaktask_import_take_command(OakTaskTask *t);
 
 int oaktask_import_footage_count(OakTaskTask *t);