refactor(task): de-Qt oaktask base + project tasks, wire codec task submitter

- Task base: Qt signals become lifecycle listeners (the async-command
  callback exception), cancellation uses oakrender's OakCancelAtom C
  handle; TaskManager runs std::thread workers, no signals
- ConformTask/ProxyTask implement oakcodec's submit-callback contract
  (synchronous interim); register_codec_task_submitter() closes the
  conform/proxy loop - conform of demo.mp4 produces pcm caches in tests
- ProjectImportTask/ProjectLoadTask/ProjectSaveTask over the oaknode
  C ABI; image-sequence confirmation becomes a facade callback
  (default: not a sequence)
- C API additions needed by oaktask: oaknode serializer file-level
  save/load (auto-initializing), footage video-params/cancel-atom/
  as-node, folder add-child command factory; oakcodec decoder
  conform_audio + image-sequence helpers; oakrender cancelatom
  get_native
- fix double-ownership: submitting a task to the manager transfers
  ownership, freeing its handle only releases the wrapper
- C ABI in include/task (task/manager/project, OakTaskTask opaque
  handle + lifecycle subscribe), every function tested (103 cases in
  build-oaktask incl. regressions)
- RenderTask family and OTIO tasks follow in separate commits

include/node/folder.h

@@ -22,6 +22,7 @@
 #define OAK_EDITOR_NODE_FOLDER_H
 
 #include "node/error.h"
+#include "undo/undocommand.h"
 #include "node/project.h"
 
 #ifdef __cplusplus
@@ -73,6 +74,19 @@ OakNodeNode *oaknode_folder_child_at(const OakNodeFolder *folder, int index);
  */
 int oaknode_folder_add_child(OakNodeFolder *folder, OakNodeNode *child);
 
+/**
+ * @brief Borrowed cast from a folder handle to its node handle.
+ * NULL for NULL.
+ */
+OakNodeNode *oaknode_folder_as_node(OakNodeFolder *folder);
+
+/**
+ * @brief Create an undoable FolderAddChild command. Owned; free with
+ *        oakundo_command_free().
+ */
+OakUndoCommand *oaknode_command_create_folder_add_child(
+	OakNodeFolder *folder, OakNodeNode *child);
+
 /**
  * @brief Remove `child` from `folder` without deleting it (live,
  * non-undoable; executes Folder::RemoveElementCommand::redo()).